Wow....that is indeed astonishing, I hadn't heard that before.
I suppose it may be because during the Cold War, the Soviets were perhaps madly and frantically trying to catch up with us in other areas, and so new R&D for lifting rockets had to be put on the back burner ? Particularly when they already had one that worked, a new model may have been difficult for them to justify to their Kremlin bosses.
Considering that the SR-71 Blackbird was a 1950's vintage design and was only retired from service a few years ago, it makes me wonder what's been going on in those secret aerospace research labs for all this time......
