Because if the way social housing is allocated the main people the bedroom tax will hit are couples who have lived in the same house for years and have raised a family who have grown up and left and now they have to either give up the family home or be worse off financially which is just plain wrong in these cases when on the other hand they are cutting tax for the highest earners.
Its subsidised housing, why should the taxpayer subsidise people living in houses with more bedrooms than they need? If their kids have grown up and moved out why don't they exchange down to a smaller property? there are plenty of families in social housing that don't have enough bedrooms...
If you want the luxury of extra bedrooms go buy your own house or rent privately and pay full market rates, anyway the amount being charged for extra rooms is still only a fraction of what you'd pay for a room in the private sector.