Rental price often doesn't correlate with house price as closely as one would assume. RTB has removed a reasonable alternative for the average person (you have to be a single mum of 8 in a wheelchair to get a council house now) and therefore landlords are left without competition lowballing them in 'decent areas' to live. RTB didn't create the housing crisis (not building houses did) but the lack of a decent social housing stock now has let landlords tear up the rule book and charge whatever they want really.
Forcing councils to sell houses for less than they were worth, taking most of the money away from them and then saying 'we think you should build another one now' is a nonsensical housing policy on all fronts.