It's probably a mistake to imagine that GAF is one mind, and it's probably worth thinking harder about the fact that so many GAFfers who focus on politics don't think Bernie can win or that he's the best choice for advancing progressive/socialist politics.
I know it's popular to say that GAF isn't a hive mind, but there's definitely a kind of groupthink or consolidation of opinions that takes place. It's why people are able to say that GAF is very liberal. There's more opposition when voicing conservative opinion that pushes those people away from political discussions. Similarly, it's easier to voice a liberal opinion and meet agreement so those people are encouraged to participate more. Over time you get these groups which each circle around a single predominant narrative. For example, we have BernGAF and HillGAF as people like to say. There's definitely a tone that each group has, and a particular set of talking points that resonates within each group, giving the group an identity beyond the sum of the individual members.
As you mentioned, I think the big reason that there's a hostility to Bernie's campaign is because people care a lot about winning. People who follow the political process on a regular basis probably do so because they have some interest in the horse race. Bernie is an obstacle to winning, clear and simple. This is the argument that gets repeated frequently, that Hillary is the better choice because she's more electable. It ignores that politics happens on a social level as well, outside of seats in government.
I don't actually think Bernie Sanders would make a very good President. He, like Elizabeth Warren, is an ideologue with a narrow view of how the world works. He probably wouldn't do well in a position of power when reality tends to deviate from expectations. Much like the Republicans (and GAF), he tends to try to fit reality into a preconceived ideological framework. I'll get nervous if these are the only voices we get on these issues, but this isn't the predominant criticism of Sanders.
I'm rooting for his candidacy because his success, even if he doesn't win, is able to bring change to the political discourse and the Democratic party. This is where I'm somewhat disappointed, that others seem to be so hostile to his movement when I feel they should be glad that he's able to do so well. And that's where I start to suspect that some aren't as on-board with socialism as they would like to believe.