When a new power takes control of a country the power structure does not really change. There are centuries of governmental development that happens far down the pyramid from the leadership. CGP Grey’s video “Rules for Rulers” touches on this in a big way but keys to power are not necessarily people but organizations. Revenue Agencies and the collection / processing of taxes, for example. The way that civil infrastructure is carried out is another great one, which involves thousands of people and hundreds of independent contractors.
Russia has had centuries and centuries of one golden rule: you can only rule with an iron fist. Weak rulers do not last long, and there is an extremely competitive and extremely corrupt pool of powerful people that are more than ready to take your place. When a regime changes, those rules do not change with it. Russia’s governmental system has also placed a disproportionate amount of control in authoritarians compared to the typical democracy, which makes the system much more vulnerable to individual corruption rather than organizational corruption.