If you think about the Democratic parties base, two groups stand out. Lefties (like most people on Lemmy) and people of color, particularly black women. If you select Newsom over Harris you i.) deal an enormous insult to people of color, and ii.) don’t go nearly far enough left to satisfy Lefties. What part of the base would be enthusiastic about such a milquetoast replacement?
It is definitely also about age. Sanders is too old to be president. Too much of a risk of mental decline over a 4 year period. Feinstein as a Senator was terrible. Feinstein as a president would be an outright disaster.