Hillary is already strong with women voters, she needs help convincing men. Warren does nothing there.
Hillary is already viewed skeptically by moderates because she is so liberal, so the way to appeal to them is to put someone *even more liberal* on the ticket?
Both of them are the same age, so it does nothing demographically.
Both are white, so it doesn't do anything for Hispanics, minorities, or blacks.
Massachusetts is already one of the most reliably blue states, so it doesn't put any electoral votes on the board. Worse, if Warren resigns her Senate seat, her replacement is appointed by the Republican governor of Massachusetts. Imagine Hllary winning, bringing along a few Senate seats, only to have a 50/50 split turned into a 49/51 count because Warren had to give up her seat.