I think Trump is right on this one.
Warren has a good message, but ultimately people don't usually think about politics; they feel politics. And she just doesn't quite have the ability to sway people the way Trump does.
Exactly this, I like Liz Warren and I think she could be a phenomenal president, but to be president you first have to get elected, and the last thing Democrats want to do is risk Trump's reelection. Warren does good work in the Senate and really unless Charlie Baker gets unseated in 2018--which isn't likely at this point--there's no reason to give up her Senate seat like that when there are better options.
Sherrod Brown, Al Franken, some random dark horse, are all safer bets anyway. Though to be fair, Warren still absolutely has the potential to win and she's miles better than Cuomo or Booker.