Foreigners won't go to Russia to file a complaint to police. Without a complaint, there is no reason to investigate anything. I think this is the explanation.
Also it is 100x more difficult to make Russian pay for something, including a ransom. So attacking fellow Russian is a high-risk, low-return move.
In the past US LE has tried to work with Russia to arrest ransomwarw groups but it didn't work out. Russia demands extradition of political prisoners or some such in exchange so it falls through.
Also it is 100x more difficult to make Russian pay for something, including a ransom. So attacking fellow Russian is a high-risk, low-return move.