Because they're ideologues.
And its impossible to discuss anything with an ideologue.
How are you going to exchange ideas with someone who thinks any opinion other than theirs is uninformed and ignorant from jump?

"It's about an attitude, the sense that righteousness excuses you from the need for hard thinking and that any questioning of the righteous is treason to the cause."

And the irony is that Bernie supports are more uninformed and ignorant of both the political process and Bernie's record than anyone.