Mostly because she doesn't have any name recognition, and equally because she is the most anti-war politician in the whole country. The think-tanks and ex-generals/CIA will be united like never before to stop that ticket, because her name (alongwith his) signals a very different foreign policy than the bipartisan consensus of the last few decades. It would easily be the most leftist candidates from any major party, I think the only comparison is FDR+Wallace in 1940.
It will also not be hard for Republicans to attack
her past.
Again, this would be my dream ticket, and Bernie went to campaign for her re-election this time so they're probably close, but I don't think it'll happen.