Rent Act 1977 (and its predecessors) gave tenants extra rights. That deterred private landlords.
Housing Act 1988 reinvigorated private sector lettings industry, by rebalancing L's rights- more even handed now.
Income tax relief used to help house mortgagors (borrowers), and abolition of old Schedule A income tax meant no ongoing tax payabe on one's only or main home.
As most people want to own/occupy, because potential for large tax-free gains, new-build-for-let market is a thing of the past.