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Basically Bernie Sanders used the wrong term and it stuck. For some unfathomable reason, he refers to himself as a socialist while espousing policies that are clearly social Democrat in line with most European states. He has done himself no favors. If he had said social democrat, he wouldn't have received as much McCarthy-ist style attacks. He is definitely espousing a European style model rather than a Venezuelan style model. Because of his mistake, the term's meaning has changed, and now others like Alexandria Ocasio-Cortez are calling themselves socialists too even though they're not.
The DSA themselves also describe themselves in terms that sound a lot more like social democracy than democratic socialism.

Bernie's key policy positions are all Medicare For All, lowering prescription drug prices, a jobs program to shift America's energy production rapidly towards green energy, and tuition free public colleges. I could source this, but he says it in literally every speech or interview he's ever done, so that would be a bit redundant!

Bernie Sanders gave a great interview in 2006 with Democracy Now where he explained his version of socialism. You can compare that with the DSA position linked above.

Why is it that Bernie Sanders always called a "socialist"?

I think getting a quick history lesson to get some context might help explain how "socialist" is used in US media.

First, I want to highlight the long history of anti-(anti-capitalism); second, show how anything anti-capitalism is generally conflated; and third, how this is the case in the present day.

Discussing communism might seem like a detour, but bear with me for a moment. I'll start with The (first) Red Scare (1917-1920):

Political scientist, and former member of the Communist Party Murray B. Levin wrote that the Red Scare was "a nationwide anti-radical hysteria provoked by a mounting fear and anxiety that a Bolshevik revolution in America was imminent—a revolution that would change Church, home, marriage, civility, and the American way of Life".

Around the time of the second world war McCarthyism (~ second red scare) took hold:

During the McCarthy era, hundreds of Americans were accused of being communists or communist sympathizers; they became the subject of aggressive investigations and questioning before government or private industry panels, committees and agencies.

You might notice that the above two quotes only reference communism. I'll discuss this a bit more below, but I want to continue the walk through history with a quote from wiki page on the history of socialism in the United States:

COINTELPRO [(1956–1971)] was a series of covert and at times illegal[189] projects conducted by the United States Federal Bureau of Investigation (FBI) aimed at surveying, infiltrating, discrediting and disrupting domestic political organizations[190] FBI records show that 85% of COINTELPRO resources targeted groups and individuals that the FBI deemed "subversive",[191] including communist and socialist organizations; ...

The point is, there's a long history in the US of government responses to anti-capitalist movements, as these are generally not viewed favorably. There's a lot more that could be said (labor movements; class; race & racism; gender, to name a few related issues), but that's a rough overview.

The second point I wanted to address was the conflation of anti-capitalist terms. I pointed out that two of the quotes only mentioned communism, but actually all of the above wiki pages can be found on the history of socialism in the US wiki page. I wanted to bring this up to point out that the government response conflates anti-capitalist movements. The same wiki page notes that

The widespread use of the word "socialism" as a political epithet against the Obama government by its opponents caused National Director Frank Llewellyn to declare that "over the past 12 months, the Democratic Socialists of America has received more media attention than it has over the past 12 years"

Even though the Obama administration and most leftists agreed that his administration was not socialist. The source for this claim is a Chicago Tribune article, which goes on to say:

To most, socialist policies are synonymous with any expansion in government spending (although many capitalist nations funnel more of their gross domestic product through the public sector than the U.S. does). source

Here's a foxnews article which defines socialism as communism, (though I think the author was trying to distinguish the two):

There was a time in American politics when the term "socialism" conjured up images of the Cold War, the U.S.S.R, Joseph Stalin and Karl Marx, nuclear threats, government domination of private industry and gulags. For most Americans who came of age during the height of the Cold War, who remember being taught to hide under their desks at school, socialism – like communism – has long been a dirty word. source

In summary, in the United States, pro-capitalism is the majority view (to varying degrees; this means different things to different people). That is, people endorsing communism, socialism, and democratic socialism are a small minority (i.e., as a ruling system of government). The distinction between communism, socialism, and democratic socialism is generally lost on anyone outside specific academic groups, and lost on the general a-politic public. It is only when you venture in to certain leftist groups that the distinction between these groups is recognized. (And maybe certain non-leftist groups too. The point is, a small minority.)

Third, to go back to the original question:

Why is it that Bernie Sanders always called a "socialist"?

You're right that Sanders is not a socialist. But he's aware of the US sentiment of the term, as I outlined above. For example, an article on a Sanders speech :

Throughout the decades, he [Sanders] argued, "socialist" has been a term conservatives deployed when Democrats do something popular:

Almost everything [Roosevelt] proposed was called "socialist." I thought I would mention that just in passing. Social Security, which transformed life for the elderly in this country, was defined by his opponents as "socialist." The concept of the "minimum wage"—that workers had to be paid at least a certain amount of money for their labor—was seen as a radical intrusion into the marketplace and was described as "socialist."

He has embraced this view of himself as someone that supports progressive social policies. Though he does tend to describe himself as "social democratic" when discussing the topic, and seems to be aware of the different in terms:

Sanders: Mitch McConnell, the Republican leader in the Senate, often criticizes President Obama, incorrectly, for trying to push "European-style socialism," and McConnell says the American people don’t want it. First of all, of course, Obama is not trying to push European-style socialism. Second of all, I happen to believe that, if the American people understood the significant accomplishments that have taken place under social-democratic governments, democratic-socialist governments, labor governments throughout Europe, they would be shocked to know about those accomplishments. One of the goals of this campaign is to advance that understanding… source

In summary, Sanders is called "socialist" because the US media in general describes policies that provide some kind of assistance (for example, food aid or health care) as socialist. This is just the current nature of the discussion at the national level. This is a rather US centric (i.e., isolated) understanding of the term "socialist" which is more properly understood in countries to the left of the United States.

Sanders doesn't seem to object to the term if it is applied to him, but when discussing his views will refer to himself as "democratic socialist" instead of "socialist." And just as the Democratic People's Republic of Korea is not democratic, neither is "democratic socialist" the same thing as "socialist." But as detailed above, this distinction is rarely made clear in US media.

Folks who support the view that Sanders is actually socialist frequently point to this quote from him way back in 1987:

Democracy means public ownership of the major means of production, it
means decentralization, it means involving people in their work.
Rather than having bosses and workers it means having democratic
control over the factories and shops to as great a degree as you can.

Sanders continues to demonstrate admiration for Eugene V. Debs, the most prominent Socialist candidate in the history of the United States.

But yes, if we focus on his major policy proposals and imagine Sanders in a multiparty parliamentary democracy, the label "Social Democrat" would be a better fit.

It seems likely that people call him a socialist because he's self-identified as one on multiple occasions.

and

When he first won election to the House in 1990, Sen. Bernie Sanders (I-Vt.) embraced his political identity. "I am a socialist and everyone knows that," Sanders said, responding to an ad that tried to link him to the regime of Fidel Castro.

_{^{Washington Post: Bernie Sanders is an avowed socialist. 52 percent of Democrats are OK with that.}}

etc.

I think there's an important distinction to be made in the examples you noted.

The left-of-center news outlets actually make an effort to call him by his chosen name of "Democratic Socialist."

• 
  The Hill (left-center)


• 
  The Nation (left)

It's the right-wing sources that conflate the terminology.

• 
  Daily Mail (right, questionable source)


• 
  Newsmax (right)


• 
  Fox News (right)

(Of course, by European standard, all of these sources would probably be considered a little more to the right than the US ranks them.)

The right-wing likes to use the word socialism as a pejorative, capitalizing on how little Americans know about socialism in the non-pejorative sense, and how much disinformation and conflations has been made about it in the past. If you live in the US, go ahead, try asking random people you know what socialism is. You'll probably get some really funny answers, like "Putin is a socialist" as I do when the topic comes up.

The meaning of the word socialist changes by country and by decade. Social democratic is especially crucial in Germany because of the national-socialist catastrophe. The word is a popular ideal which was abused by despots and which lost favor after the 80's, mostly for it's use by despots, it's vagueness and lack of concencus.

Socialism in the 20th century started out as an ideal term for debate of human rights of expression, minimum wage, trade unions, in the 1920s.
The 1920's ideal is how I was introduced to the concept of socialism in philosophy and history, and same goes for Bernie Sanders too, probably.

Socialism changes a lot by decade and Bernie probably associates socialism with the ideals of his father's generation from the 30's, rather than the recent historical corruptions of socialism used by Venezuela and Germany.