Or maybe Clinton and Biden figured that they would win the primaries without airing the red-baiting oppo and it would be better for their chance in the general if they kept a lid on the worst and most blatant hippie-punching.
In a left-leaning state, you might well be able to pick up as many votes as you lose by running as a self-described Democratic Socialist. Not necessarily true of the country as a whole. Thus, the attacks you use in different contexts change.
Yes, he doesn’t have the experience of running a large and politically divided state that Joe Biden got while being senator of… (checks notes) …Delaware.