Short answer. The people who already live there don't want to be neighbors with those sorts of buildings and any politician that suggests it will be voted out of office.
> A counterpoint - You then get jerry built high rises built to minimum standards and smaller rooms which leads to tragedy's like Grenfell tower.
The old socialist buildings are made form reinforced (rebar) concrete... even the inside walls... and outside walls.. and sometimes even balcony ledges. Even occasional gas explosions usually just blow out some windows.
Short answer. The people who already live there don't want to be neighbors with those sorts of buildings and any politician that suggests it will be voted out of office.