2. In the end, far left does not have many places to go to. At worst, they would vote Stein or stay home. Most of them will vote Clinton anyway. It is the mainline Republicans and centrist independents Clinton has to be appealing to. Warren is as disliked among those as she herself is, or more. Choice of Warren would help those parts of the electorate in justifying their vote for Trump. Kaine would not provide that justification.
When it comes to most winning tickets, it's actually the other way around. Jay Heinrichs, a rhetor, actually detailed this before. Winning candidates position themselves in the middle, rather than their VP. That's why they choose a running mate with more extreme opinions of their own, to energize all factions of the base.
It's what you saw with Nixon and Agnew, Clinton and Gore, Bush and Cheney and Obama and Biden.
