I was hoping for Gretchen Whitmer or Tammy Baldwin, or if there really was some kind of requirement that it had to be a Black woman, I would have preferred Susan Rice. That said, I think it would’ve been a horrible mistake to pick Elizabeth Warren, Karen Bass, or Tammy Duckworth, so I’m relieved we dodged those bullets and overall I’m fine with it.
Perry Bacon Jr. on the 538 Podcast said a few days ago that all the vocal Black figures who kept doing TV hits and writing columns saying Biden absolutely had to pick a Black woman actually meant he had to pick Harris because they didn’t see any other Black women as truly qualified (none are senators or governors) but they couldn’t say it out loud. So it may be that the fix was really in on this more than we ever knew.
I think Biden originally wanted to take Klobuchar but then when she was out he wanted to take Whitmer, and then when everyone insisted so strenuously it had to be a Black woman he wanted to take Rice, but in the end felt he had to take Harris.
I have not seen anyone, in all the voluminous “veepstakes” commentary I have read/heard/seen, note the (unintended) influence of two men in leading Biden to this decision:
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Bernie Sanders. I firmly believe the main reason Biden made the promise to pick a woman at the last Democratic debate was that he knew the race was almost over and he wanted to pre-empt any chorus from the left that the only fair choice would be second place finisher Bernie. Some pundits have said he needlessly boxed himself in, but IMO they are missing the fact that he wanted to be boxed in! Or at least to have Bernie boxed out in a way no one could criticize him for. Pretty clever.
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Derek Chauvin. I’m sure Harris is as repulsed by Chauvin as most of us are, but I’m convinced that if he had not knelt on George Floyd’s neck, we would have a Biden-Klobuchar or maybe Biden-Whitmer ticket.
David Axelrod believes it was actually the Biden campaign that leaked Gretchen Whitmer’s interview, as a trial balloon. If that’s true, we can add one more factor to the list helping Harris: the swarm of angry commentary we got from the Black chattering classes over the prospect of Whitmer. So even close to the end of the process, they may have been really wanting to take Whitmer (who I think would have been the smartest pick), but they needed to feel like there was only a low rumble of dissent rather than an angry roar. Of course, they got the latter, which may well have been decisive in scuttling Whitmer. If so, I guess I have to tip my hat to that group of chatterers for really exerting influence at a crucial moment. Unlikely we will ever know for sure, but it’s a huge pivot point in history so I hope we find out more.
Paul Begala pointed out that the qualification for becoming a Democratic veep has, for the past 40 years, been running for president yourself first and failing colossally. 
(Let’s just hope that this pattern does not continue to ultimately losing the presidency in the electoral college despite winning the popular vote.)