There is an answer to this question, it is multifaceted, it consists of many factors.
Firstly, it is certainly a huge country with a large population, unlike many European countries. In many Asian countries, poker is either banned or has their own
poker sites. Many European countries have their own poker sites, including PokerStars. Therefore, not all countries are represented on PokerStars. And also many big countries like USA, China etc. do not play on PokerStars.
Secondly, during the Soviet era,
gambling was strictly prohibited in Russia (USSR). But there have always been a large number of other players (not just poker) who gambled secretly. And in the evenings, especially on long winter evenings (do not forget that winter in Russia, especially in Siberia, is long), many played card games (not gambling), mainly children and young people. Since there were not many other board games, there were no board games like Monopoly, etc., there were no computers and the Internet.
Thirdly, in Russia and other countries of the former USSR there has always been and is a highly developed logical thinking and exact sciences.
Fourth, the income of the majority of the population is small, and any winnings in poker allow you to live on this money.