There is also a fair bit of "common sense" involved here. Comparing the territory, population and economic concentration of the USSR proper (pre-1939) to present day Russia, Russia is the closest match for a continuator state.
Russia is also where a lot of the fighting took place that beat Germany, which was certainly a practical part of what originally grant UN Security Council veto rights: being a WW2 victor.
The other USSR constituent states were all much smaller in all those respects.
And that's before one gets into who had practical control over most of the nukes given up by the 3 Budapest Memorandums as well as the overall USSR military.
People in 1991 mostly didn't expect Russia to devolve into what it's become today. Many Western Europeans were overjoyed to see their cousins "come in from the cold", ditch Communism and become a normal state. Not so much those who had had direct experience of Soviet rule, maybe.
Why should they have antagonized Russia by withholding this transfer?