Basically Gary Miller's newly-redistricted seat seemed like such an easy pick-up for Democrats in 2012 that it drew too many candidates into the race, and then they proceeded to split the vote too many ways for any one Democrat to make it into the top two. So Gary Miller got fucking reelected! Miller's retiring this session, but it'll probably happen again unless the CA Dems get their shit together.
Anyway, the point is that every Democrat in a seat more liberal than a D+5 has basically no chance of being unseated by a Republican, and they can all vote like Bernie Sanders if they really want to. But too many of them don't want to so we can't have nice things.