Starting around 1975, housing supply stopped keeping up with the growth of housing demand. It picked back up fifteen years later, around 1990: https://www.brookings.edu/research/the-goldilocks-problem-of...
(But that is on a national level: in the West, in comparison, housing growth hasn't been sufficient to house the new households formed at any time in the last 70 years and so the problems run deeper.)
Normally, cheap housing is housing that was expensive housing 30-100 years ago. Think of all of the post-war apartments in NYC, or the Boston multi-family houses built around the previous turn of the century.
Since many cities currently lack the recently-aging housing that would normally be filtering down market right now, the elasticity at the top is affecting housing too new for developers to be out of debt on, and thus we are unlikely to see it translate into lower prices on the low end until another 20-25 years pass and we get out of this housing doldrum.
(Not the West, though. Sorry San Francisco: you just have to build more places where people can live.)