Igor, everything is very simple. The actions of the participants of the Bialowieza Agreement were not covered by the Criminal Code, since the Criminal Code of the USSR was not. Each republic had its own criminal legislation, and with unratified norms of international agreements.
And before the International Tribunal, to consider this crime within the framework of international law, no one initiated this procedure, unlike Germany after the Second World War, Serbia, Iraq, etc. Here is the whole answer. If you look at the initiators of the processes, you will see that they have always been the United States. Now think about who on behalf of our states could initiate this procedure and why did not initiate ... ???
Look for the answer in the regulation on the International Tribunal. You will not find, I will say later, I will look at legally competent users.