Last time, for all the talk about energized young leftists voting for him, he got a lot of support from Democrats (as in, people who traditionally vote in Dem primaries and routinely vote for the Dem candidate up and down the ballot) who didn't want Hillary Clinton. He was the only option for those voters. This time, they've got more choices.

On the other hand, Dem primary turnout is going to be WAY up from 2016. And at least some of those people who sat out 2016 will go for Sanders (along with the other candidates, of course.) i think if he can get a bigger share of the new voters than anyone else and the voters he loses divide fairly evenly among the other candidates, he could win. But it's not going to be easy for him, he'll need to make a case for himself that isn't just a rehash of 2016 that also sets him apart from the other candidates (again, none of whom will have Hillary's baggage.)