It's simply because of deep poverty of vast majority of population and unavailability of contraception. Living standards in the Soviet Union were on par with th

It's simply because of deep poverty of vast majority of population and unavailability of contraception. Living standards in the Soviet Union were on par with third world, not the first: massive industrial output did not go into consumption, but was either directed to the military, or wasted. Plus, total fertility rate was rather high in 1970s and 80s (around 2.5) only with inclusion of Asiatic republics where people effectively lived in prehistoric state with little change over 2000 years, with totemic idols replaced with Lenin's statues. In Russia proper, it was hovering at or slightly under the replacement ratio.