My understanding was that it was the tenant rights movement that killed SROs and boarding houses by making it practically impossible to keep them orderly, because it made eviction almost impossible and compliance with anti-discrimination laws presented too large a burden for low-cost housing.
And rather than being refuges for same-sex couples and generally "[offering liberation from family supervision and the constraints of Victorian mores", they were the opposite -- often being extremely stringent in "morality" clauses and forbidding mixed company after dark. They were frequently racially exclusionary in ways that became incompatible with civil rights laws.
The reality is that the situation was probably a mix of both attacks -- attacks through over-regulation and tenant rights, as well as direct attacks on SROs as hotbeds of crime and illicit or immoral behavior, but I'm curious as to the mechanics of how this came to be.