Actually, I've been in the position twice when I was renting when the landlord offered me the flat for sale. In both cases, while it suited me to rent it, I didn't want to buy it as I didn't think it was a suitable property for me/us long term.
I wouldn't want to be in the position when private rentals were effectively banned and have a choice, get a mortgage and buy this flat or another now or be evicted with nowhere to go to (no private landlords = only the council properties for rental and suddenly a huge number applying).
where do you see people living who can't get a mortgage? People like those who've defaulted on debts and now have a poor credit rating. People with no savings for a deposit. Unemployed people who need housing benefit to pay their rent (which they can't use to pay a mortgage). People who have just graduated and are moving to a new town to start a first job, need to live somewhere for their first day, but cant get a mortgage until they've got an employment history and some savings? People who've just been offered a 6 month contract in an amazing sounding job at the other end of the country, but don't want to sell up the family home and shift the kids out of school for such a short period... You know, the people who rent privately now and it suits them.
There's a lot to be said for putting greater tax and restrictions on BTL to stop it being quite such an attractive financial option, but don't assume everyone who is renting wants to buy that property in that town, or could buy it even if the price fell.
Don't assume that everyone can afford to manage a property either, paying for maintenance as well as a mortgage - it suits a lot of lower paid or unemployed people that if the boiler breaks, someone has to fix it.
basically, don't assume that a change that would benefit you a bit wouldn't fuck over a lot of other people.