So kinda "we already have a program for this" and kinda "it infringes on state sovereignty by letting local government review our decisions" but it closes on "it costs money" which seems like the main reason.
"The veto of #AB309 – revenue neutral cost rental social housing on public land which could be built starting next year – while we only commit to study the issue for another 2-3 years is an insult to the millions of housing cost-burdened residents of California."