Because despite his being an Independent for most of his career the guy has been a reliable part of the Dem caucus for just ass long.Why would Sanders listen to them?
And he has stated over and over again that he doesn't want to see a Republican win the WH and all his staying it all the way to the convention will do is hurt the Dems chances in the general.
Hillary has more votes because places like California havent voted yet and Alabama, Texas and bunch of other places irrelevant to progressive values have. Use some common sense.