electoralism

"And because the president endorsed Kamala Harris immediately, that really made it almost impossible to have a primary at that time."

IMO, it was the money, not the endorsement. Biden had ~$100M on-hand when he dropped out. There was a legal challenge about the money being used for The Harris campaign and the Dems argument was that since Harris' name was already part of the ticket, they wouldn't have to refund it.