But he is, at least socially. I'm not sure how the other two are better choices, economically anyway. Trump seems to have that air of "pro-wealthy" about him, as well as few bankruptcies.
Why do you think he is the better candidate socially?
He has some ideas, some better, some worse. The way he behaves now indiciates that he does not understand the political game very much. So it is questionable whether he would ever be able to to even properly execute his ideas.
It is rather probable that he is unable to find compromises, while clinton is good at that.
Clinton understood that at some point you have to back down, you could see that in the primary against obama.
She was then a pretty loyal supporter of him. She put the parties behalfs before her own.
Sanders is no real democrat, he just ran in that party because he has no chance as independend (which is sad, but that is how it is).
Looking at the fact how little he seems to care about that party now indicates he would not even have been able to get his whole party on board with his ideas, which would be even more problemativ.
So you'd have a president with no knowledge (and no ambition) in foreign politics and who would probably not get anything done in inner politics.
Clinton however might not be as progressive as he is, but she is still one of the more left leaning types in the party.
It is more probable leftist reforms are made under clinton than under sanders, and this is why she ultimately is the better candidate.