Basically because Stalin insisted. When the UN was being formed during the second world war, the major powers were jockeying to increase their influence in it. The main thing to come out of that was permanent membership of the security council. In a different attempt to get more influence, the USSR claimed that, since it was a union rather than a single country, all of its constituent parts should have separate membership - the US then suggested that every single state should have its own seat on the same basis. Obviously the USSR didn't think much of the US counter but they still insisted on some extra seats, so the eventual compromise was to give seats to Ukraine and Belylorussia as well as the main Soviet/Russian one.

As for sovereignty, UN membership implied soverignty in the same way that communism implied a workers paradise - maybe in some sense, but not in the USSR.

Sources: Hitler and Stalin Parallel lives, by Allan Bullock; Russia's War by Richard Ovary, The Penguin History of Russia by Robert (?) Sercice; and the Rise and Fall of Communism by Archie Brown.