It's debatable who the best player on the Warriors is, but being the best player doesn't make somebody an alpha (nobody would call Smart our best player, but I don't think anybody would disagree that he has an alpha personality). In OKC, while Westbrook is certainly a take charge kind-of player, Duant still felt he had too much responsibility to bear; it was really just the two of them.
In GSW, Durant has been able to slide right in and play with a bunch of other all-stars. He has performed well, but has never had the pressure of absolutely having to play well...that is until Curry went out and the Warriors 'struggled'. I would love Durant on the Cs more than just about any player in the league, but he is absolutely a beta...and also, who really cares? He was able to find a situation in which he felt comfortable and now he gets to win lots of championships without all the pressure. That has to be pretty cool for him.