Its because there is a vocal minority of Bernie supporters who say they're Bernie or Bust and people love to paint all of us with the same brush. Its a narrative thats been established since the 2016 election that all the Bernie supporters would rather take their ball and go home if he doesnt win the nomination which is frankly not true.

However, be it the Trump campaign, opponents campaigns, news orginizations, or even just vocal supporters of other candidates, its an easy anti electability argument to push. And there probably are some who would do exactly that, just to be clear- but Im sure there are supporters of any candidate who feel that way.

Long rambling explanation aside, what I think it really boils down to is that its easier to attack loud asshats on twitter and reddit than to attack Bernie on issues so we see it a lot more.