@Lash,
Think what would happen if she endorsed Clinton, and then Sanders won the nomination. A lot of people would look at Warren as a possible running mate. Clinton can't pick Warren as a running mate, because Hillary worries too much about what Wall Street thinks. But Bernie wouldn't need to have any such qualms. He's already pissed off banks as much as is humanly possible.
So let's say Bernie picks her to run with him and she accepts. How does Warren then justify having endorsed Clinton earlier? Does she say something like "oh I just didn't think Bernie could win"? Nah, she's his pick for vice president at that point. She can't say that.
But that's just my theory of course, which occurred to me when Calamity Jane mentioned her as a possible VP. The truth could be a lot simpler. Warren could take Ed's view of things--that it's Bernie now or no one ever.