1.Poor maintaince of actual buildings
2.Soviet poor planning of districts, which absoluely not much modern realies, weather, alive human beings activisites etc.
3.Often cheap construction of this buildings and not very good construction projects in therms of planning apartaments.
4.Very poor maintain of communications.
5.Population of such buildings.
Modern buildings are even worse, since it's litteraly human hive with cheapest materials possible, no managment of space around buildings at all, often 15 and more floors.