The obvious reason is that to a first approximation nobody is building public housing, and so appeals to public housing, especially from owner-residents, read to housing activists as a dodge, a way to perform advocacy for housing without actually providing housing.

A subtler reason is that public housing is drastically more expensive to build than private housing. There's multifarious reasons for this, and the underlying problem is absolutely worth solving (it's one of the three big prescriptions in Abundance), but it's a bigger problem than housing. Right now, people are focused on whatever's going to bring the most units online fastest.