The real galaxy brain moment here is that there's a window of time after a Democratic politician goes "over the top" (to borrow a WWI turn of phrase) during which undecided voters evaluate them on their own merits, before right wing media outlets can calibrate their smear campaigns and really start eating into their favorables. Kamala's been out of the public eye for long enough that the clock is at least partially reset. AOC, on the other hand, is The Devil Herself to the right and they've been attacking her for years. Better to pull a rising star who is unknown enough to similarly benefit from the right-wing hate machine's warm-up period, and being enough progressive bona fides to the ticket to cushion some of the criticisms of Harris's prosecutorial history.