San Francisco has a large transient population of renters. They drift in, party and work hard, and then leave when they have kids or want to settle down. This is because San Francisco is not very family friendly - it's a city with more dogs than kids -- with more drug addicts than kids. It's much more of a party town than a place where people can put down roots. The average renter lives in San Francisco for 6 years and then moves away. Don't think of San Francisco as a self-contained unit, it is a 7x7 mile square containing 800K people in a larger SF Bay Area containing 8 million people. People go to the Square when they are in the mid to late 20s to early 30s to party and then move to other places nearby when they have a family.
This transient population generally does not involve itself too much in local issues because it is not attached to city. That is why SF mayoral candidates and city council candidates spend a lot more time discussing how they will fight global warming and white supremacy rather than discussing local zoning. Because the transient population cares about these general non-local issues and generally don't bother too much with details of local issues. This has some hilarious consequences, for example, one rich family whose view would get blocked by new development managed to put on the ballot a measure to block the development, and got it to pass in the name of "Save our Waterfront From Greedy Corporations". That's why SF has a large parking lot and tennis courts on prime waterfront real estate. There is no one easier to manipulate than the uninformed do-gooders that is the SF voting population -- just link more housing development to "greedy corporations" or "gentrification" or "large carbon footprint" or complain about lack of diversity in the construction crew or administrative staff, or new residents, etc. So many ways to kill a project.
The result is that control over local issues -- e.g. what the city council actually does -- is handed over to a small core of residents that do care and have put down roots -- e.g. to homeowners. And homeowners want house prices to rise and for neighborhoods to not change radically. Therefore the SF city council and mayor are very conservative about things like zoning, even as they spend a lot of time espousing radical positions on generic lefty issues.