In my experience (which is admitttedly mostly anecdotal, but comes from having attended a number of public forums/planning meetings etc.) another huge group of people fighting hard against new development are the ultra-left wing of SF local politics. These people aren't against new housing per se, they are just rabidly anti-developer. They spend so much time delaying projects by trying to get developers to increase the amount of below-market rate housing in their new developments that the developer eventually just gives up because the whole process takes so long and the potential profits dwindle down to nothing.