After all his rhetoric about her corrupt campaign financing, and the corrupt democratic party, to concede to her would be like joining the evil she is owned by. Bernie would be capitulating to the corporate greed that got her the nomination. It is Sanders obligation to endorse a third party instead of Shrill.
Bernie feels that it is far more important Clinton win than Trump.
Simple as that.
If Bernie endorses Clinton, his supporters will be disillusioned with the democrat party as I have been for over twenty years. Most, like me will vote for the Green party candidate, and Bernie will go down in disgrace. There is no difference between Trump and Clinton...they are both war mongers...both corporate flunkies....both narcissists... and neither will fix anything for the next four years...so Bernie has nothing to lose by endorsing a third party.