I see a lot of people ask: "why not use this propellant", (please don't read this in a passive-aggressive sarcastic voice) and they forget that the Soyuz / R7 was THE FIRST space faring rocket. While they have upgraded it, if they were to have done something as drastic as changing propellant or something else, they probably would have renamed it. I notice the fact of the Saturn rockets and that is because they were only for one program (technically two because of Skylab, but it probably wasn't even a though before the last few Apollo missions.), so it would make sense to keep the name. However, the reason the Falcon 9 was able to be so revolutionary was because it was designed from the ground up. Modern day Delta and Atlas rockets a pretty different from their first variants, but what keeps some of them limited is that they are still based on that 50-80 year old technology. Also, Russia does not have the same funding as NASA or ULA. They haven't made any new rockets since the fall of the USSR. It has all been Soyuzs (Soyuzi? Whatever the plural of Soyuz is.) and Protons since then. Come to think of it, that is all they have ever used. In the end, I think the main reason for the continued use of hot staging is funding, or lack there of.