Comment by manfredo

The cause is that more people moved into the city than there were housing units constructed. It isn't an either-or question, it's both. San Francisco has more demand than supply precisely because an insufficient number of housing units were constructed relative to the rising population. Had the city built sufficient units to accommodate the new residents there would be no shortage of supply. Had there been no influx of residents there wouldn't be either. But we can't just outlaw people from moving into the city, so the solution is to increase the supply of housing. And NIMBYism is a big obstacle in increasing the supply of housing.