Many property owners are raking it in, and not reinvesting in the properties. I've walked past a few shoddy converted 60's office block hostels, I wouldn't house rats in them.
I don't believe there's the funding, workforce or will to start and complete a significant social housing building programme, even by ripping up planning regs. Few councillors want to go against nimbys, a recent small project was cancelled when the locals said it would create a ghetto!