Affordable housing increases the supply of housing which is bad for landlords because then tenants can just move somewhere cheaper (especially if its mandated to be cheaper by law like affordable housing). A surprising amount of (primarily white) Americans are small landlords who own a second or possibly third property that they rent out. They make up the vanguard of the "home owning" class, Americans who own a home which is by far their most valuable asset. Most of these people are not landlords and have mortgages, but the fear of their most valuable asset being devalued terrifies them. A lot of this is couched in racism and NIMBYism as well, but make no mistake that the primary opposition to affordable housing comes from landlords and is class based in nature. This is the crux of the problem that the liberal YIMBYs cannot grasp, is that landlords are fundamentally opposed to new development (unless it is desperately needed) in the same way any monopoly would be opposed to new competition. The supply of reasonably priced housing on the market (different from the total supply since housing must appreciate) must be less than the people looking for homes and the landowning class exercises political power and controls the market in accordance with that law.