I uh don't think the point is or ever has been winning, and certainly isn't "uniting the Democratic Party".
I doubt Sanders ever expected to win and was presumably as surprised as anyone else that he got as far as he did. I don't think that was ever the end game. So there's no reason to quit until forced to. That would be counterproductive.
Then he is doing more damage and that is not good.
Look I dislike Clinton but it is over and staying in is just for their egos now and Sanders should be a true leader and after California say goodbye.
Hell I like Sanders but his time is done.
He is the Progressive Left version of Ron Paul, and he is fun but there is a time to say goodbye and let the season roll on.
The same for Cruz and Kasich but I believe Cruz is too hell bent on getting revenge with Trump over comments like " Canadian Anchor Baby " to let it go, and I really do not know why Kasich is still in unless he think he can steal it in a Brokered Convention and if so it will split the ticket even more...