What's your definition of poor? Because by the 70s most Soviet citizens had access to modern necessities like food, water, electricity, housing and healthcare, things which some Americans still don't have. Their standard of living was significantly higher than it was in the early 1900s and better than actual poor countries in the global south. They weren't as rich as the western nations, but those countries had a 50-100 year headstart on development so that's to be expected.

People will often compare the Soviet Union to the United States and point to how much less they have and blame it on communism, but that's not a fair comparison. A fair comparison would be to a mexico that also had everything north of Mexico city bombed out in the 1940s. With that in mind the soviets don't seem so poor.