I think that the reason is that if you make a not white guy get cuck the usual "genius" will pop out and label the work as racist and then goats and sheeps will just follow the lead doing the same... we all know pretty well that this type of people is always ready to generate chaos when there is no reason for do that and behind it there are others ready to follow
That... really isn't the reason
The reason as to why the white guy gets cucked is because most western media treats white people as the default, so they usually get the short end of the stick in games like these. It's also why, in Japanese NTR, usually Japanese people are the cucks. The main character is a member of what the media he is from considers as the "default race"
In regards to western games (As well as some non-western games, since one of America's biggest exports is culture through its entertainment industry), there is also this whole myth about how girls "go black and don't come back", so that also plays into the overall structure of cuckold porn. This kinda originates from some of the "They will take our women!" propaganda that used to be common, so it also plays on that entrenched fear of a more "feral" and "uncivilized" man (not saying that these things are true, but they used to be very common stereotypes that are still somewhat prevalent in some places, even if they are wrong) conquering your woman and putting you at the bottom of the social hierarchy
Tbh a lot of NTR does kinda do this, even if it comes from a different cultural background. This idea of a good girlfriend/wife betraying her loving partner because a "superior" male managed to conquer her is pretty common in not just NTR but corruption in general, either because the viewers like inserting as the bull and conquering women that are in relationships, inserting as the cuck and getting off on the humiliation, or even because they just like seeing women fall to their desires, to the point that everything else stops mattering to them
Russians are Europeans, the vast majority of their population lives in the European part of Russia and they are much closer to Europe culturally than to the rest of Asia.