It's the most beneficial for the DNC. They raise more money under Trump as a "resistance party" without appearing like they are intentionally tanking their chances. If they win, they can continue to generally do nothing and let the bourgeoisie have their way, if they lose, they can fundraise better than before and run a "better" candidate next time, ie an Obama-type.
Putting someone new as the frontrunner introduces too many variables for the DNC. They should do it, but they likely won't.
New Vegas flashbacks
Yep, time for a reinstall.