An obvious answer is basic misogyny, and of course that played a role, but I have never had a more fleshed out answer. There have long been various whisper campaigns that surfaced momentarily in the press about her intelligence, about her skills as a boss, about how she'd largely been sidelined as VP, etc.
Again the whisper campaigns themselves are standard manifestations of misogyny, but that doesn't fully explain the impulse behind them.
My interest mostly isn't in evaluating any specific criticism of Harris, it's in trying to figure out why the group chat wisdom among this set of people was that Harris shouldn't be Biden's successor (and this was true before the push for him to resign).
My basic guess is/was that either Harris truly had horrendous unfixable flaws as a person/candidate/boss/leader (I doubt this, at least relative to her peer group), or that it was a basic rival clan issue: too many people who think they do/should run things don't feel particularly close to her center of power, and if it isn't about them, then it's bad.
There isn't a lot of that now. Career concerns demand it!