The primaries were rigged, I can't find the write-up sry
I mean he continued with the dems for the obvious reason I should think. He is part of the establishment, certainly a zionist, and would prefer the cushy place he carved out for himself within the dem machinery as the lightning rod for leftist deviations to the grueling work of grassroots organizing. Imagine if bernie actually went rogue, what he would have to give up. This was the path he chose way back in the 90s and he isn't gonna give up soon.
https://m.youtube.com/watch?v=OLNQEHbusSA
If you want a deepdive into bernie and the 2016 campaign the ask on hexbear, a lot of people that organized for bernie and got disillusioned and radicalised in particular by the 2016 rigging are on there. They can likely give you a week-by-week run down of the campaign lol. Or how the dem system is rigged for the establishment.