I mean timing-wise, Kamala dropped out when she did because she was polling at 5th place in California a few days before the ballot filing deadline for the state. A poor performance in California would’ve opened the door to a left-wing primary challenger in 2022.

For additional context, left-wing primary challengers are more of a threat to Democrats in California because the state uses a system where all candidates are on a single primary ballot and the top two go to the general. This often results in the general election being between a moderate (usually the incumbent) and progressive Democrat - and the state’s Republicans sometimes vote for progressive non-incumbents out of spite.