Because of people like me that have ancient rent-controlled apartments in SF that would leave them if the rent went up. I have many friends in the same situation; we're not enjoying SF like we did in our 20s and 30s, but the rent is too cheap to move.

Right now the market is bifurcated into the lucky ones who have absurdly cheap housing and the unlucky ones that have absurdly expensive housing. Without rent control, the equilibrium state would be something in between.