Notably, the only district with a significant urban core that de Leon won is in San Diego (CA-51), where he won both counties in the district. Why are San Diego voters less inclined to vote for Feinstein than voters in LA/OC?
I was gonna assume the next question you'd ask would be "Why did de Leon lose Santa Cruz County?"
Nonetheless:
Perhaps the dominant questions to ask should be "Why did de Leon win & lose the counties/districts that he did? Provide an answer for every specific county & district"
(1) lack of name rec
(2) lower turnout from UCSC during the mid-terms (this is a major reason really if it was a presidential election he would have won)
(3) suburbs of SC (Scots Valley, Capitola) are pretty "moderate"-Democrat, more favorable to establishment Democrats.
(4) SC is very left-wing, but Feinstein is popular with Democrats (for some reason). So you would have alot of people who are left-Democrats but still vote for the party line. And Feinstein was the default choice.