The USSR at the start was incredibly progressive for the time on social issues, probably being the most progressive country in the world, why and when this changed?

When the Bolsheviks came to power in 1917 they liberate abortion (the first country to do so) and were the first and only country in Europe to legalize homosexuality at the time. But at the time of the end of the cold war, the USSR was more conservative that the US and had reverted some of these policies. Why this changed?