The reasons are pretty obvious if you think about it bunny.
Firstly, in some areas there's no shortage of SH per se or certain types of property such as high rises. There are situations and places where it's literally the working couple or leave it empty. Which would be stupid.
Secondly, ghettoisation is what happens when you make SH only for the poorest and most desperate. And two working adults is a pretty low bar. Ghettoisation and high turnover of residents combined are a recipe for all kinds of problems.
Thirdly, it would disincentivise work. If two officially working adults in the household loses the family home, people will refrain from declared work, they will work illegally and they will lie about living together. You are naive if you think this won't happen.
Fourthly, it'd be a twat to administer.
Even individually, these are more significant than a few posh, probably leased cars outside houses that for all you know are ex council that've found their way onto the private market. Cumulatively, they certainly are. This is why the Tories dropped the 30k cap idea. Because it was unworkable.