Because everyone knows who Trump is. A loud-mouth, NYC real-estate tycoon who sold himself, yes lied about, and worked his butt off to make good. A bull in a China shop. But, he didn't change his race to get ahead. Rather he employed minorities. He wasn't a politician until becoming president. She's the one who claims she only does good. She's the one looking for minority votes. Yet, she used minority status that she didn't have to advance her career. Oh sure, Obama will come out and support her like he did the Canadian PM' faux pas regarding black face. But how many minorities will see through the facade and stay home rather than vote for her? How many people will see her too far to the left and get nervous about where she wants to take the country. No, Biden was a better match against TRump. A Mr. Nice Guy, centrist liberal, who union and other workers can identify with. These men and women, these deplorables, can't ID with Pocahontas flailing her arms around lecturing everyone like a college professor from Harvard. Oh wait, she is a college professor from Harvard.
A recent New York Times poll, in conjunction with I believe Marist, found that if the vote was right now with Trump and Warren on the ballot, Trump wins in every single swing state by an amount larger than the margin of error.
That's right, a liberal news source found that regardless of this current impeachment circus, Warren looses big time to Trump. The NYTs even stated that with Warren on the ticket, Trumps electoral college advantage would be higher this time than it was in 2016.
For what it is worth, the poll also shows Bernie loosing to Trump in every swing state but one, and Biden beats Trump in every swing state, but not by an amount higher then the margin of error to any of them.
So, if Warren wins, Trump is re-elected. If Bernie wins, Trump is re-elected. If Biden wins, perhaps Trump looses, however given Biden's current issue with raising money, I still doubt he can pull it off.