Countries join NATO of their own free will, mostly because they are afraid of Russia. So the reason why Russian sphere of influence shrinks is because Russia acts like an asshole.
Russia is not entitled to any "sphere of influence" outside its own borders. Personally I would gladly cut Russia out from the rest of the world with 2000 feet tall wall made of ice, so that Russian zombies can't get out.
At the same time, as perhaps is the case in China, a significant part of the Russian population is afraid that without the autocracy, there will be disorder that will be worse than the autocracy, as various factions fight for power. They have numerous examples from recent history - including Ukraine. Yes, Russia was instrumental in making sure there was disorder in Ukraine, but who is to say that another power will not do the same thing to Russia should the people get rid of their autocracy? As I understand, the same sentiment is fairly popular in China, too.
Russia is not entitled to any "sphere of influence" outside its own borders. Personally I would gladly cut Russia out from the rest of the world with 2000 feet tall wall made of ice, so that Russian zombies can't get out.