The existing old residents (50+) were already homeowners before the SV wave, and the new homeowners (who are younger and richer) followed the same bad habits of the older generation (albeit, for some different reasons).
The controlling political power in SF are homeowners and both new and old have come together to try to keep more people out, which has subsequently caused a bigger problem than what they originally feared.
SF is extremely liberal in some senses, but when it comes to housing they're extremely conservative. It's basically the worst combination you can make in the US right now.