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.

It’s capital, political and financial. Everything costs, got to pay for gerrymandering somehow.

Vetoing costs. More than half the legislature voted for it.

It can win you a few friends but you lose more.