From what I remember, it was because Russia's age rating classifications changed, so that Stardew Valley would need to be rated "Mature" due to the possibility of same-sex marriage in the game. However, the only way to do that would be to make the game Mature in all European countries, so the decision was made to just pull it from the Russian store instead. I'll investigate again when I have the time, to see if anything has changed on that front.