Things like condition, size, location and affordability of housing, combined with changes to household make-up - especially the change to smaller and more fragmented households with more single people and childless couples, more elderly people needing sheltered or supported housing, and an explosion in domestic and foreign student numbers over the past three decades.
When I was working in Council housing in the 1990s and 2000s there were tens of thousands of houses and flats in the midlands and north of England that stood empty because Councils couldn't attract tenants (condition not great but OK) - mainly because the homes were in the wrong place - where the jobs were two generations earlier. That was at a time of steady immigration.