They might be under pressure but the taxpayer is still entitled to not have their money thrown at developers.
I think you are making two separate points really: (1) taxpayer's money is being wasted by have two housing agencies competing with each other and (2) by bulk buying developments, it reduces supply to those in the market to buy a home (presumably at the lower end).
I think both points are valid. Regarding the bulk buying of developments by housing agencies, I suspect they have been told to buy what they can to get the number of social housing up, as that's where the government are feeling the pressure. Those adversely affected (those who would have bought these houses privately) don't have much of a voice unfortunately.....