alt Hacker NewsNewsom vetoed the ban [1]
From that article:
In a statement explaining his veto decision, Newsom said the measure was “unnecessary” because discrimination based on caste is already prohibited in the state.
(Just adding context that I would have missed if not for another commenter pointing it out further down)
For whatever it's worth, that's been a consistent trend with other things Newsom has vetoed with statements that he considers the vetoed item to be already covered by other laws, including some purely technical legislative things. I think it's likely that he sees himself as trying to keep California bureaucracy from growing indefinitely, especially with his push for things like CEQA process reduction/simplification.