The Demand Side of Democratic Backsliding (cambridge.org)

To be pretty fair it is rare for there be a strong primary challenge against an incumbent. Look at Reagan '84 or Clinton '96 or Bush '04 or Obama '12. There was the strange thing that the Democrats changed horses at the last minute -- though I agree if Kamala had won a contested primary she would have been much better defined in the minds of voters and less defined but what her foes said about her.