If you want the democrats to win, choose a moderate democrat that may even share a few mild conservative views. He'll capture all the usual democrat votes, get a lot of moderate swing votes, and might even get some GOP votes that are tired of Trump.
Choose a Bernie Sanders-type and you'd get the usual democrat votes, a bump in college kids who don't vote or vote Democrat anyways, but alienate a lot of the moderate swing voters and have zero chance at capturing anyone right-of-centre.
Crazy thought, but you'd think a POTUS would want to appeal to and represent the most Americans as possible.