The veto is an extremely powerful tool on the UNSC. Since the UN was still relatively new, did the Soviet Union not realize at the time how powerful it actually was?
While more can always be said, this answer from /u/Killfile addresses your question. (The short answer is: it could have been a blunder that happened while the Soviets were boycotting due to a kerfuffle over China, or it could be Stalin was intentionally wanting to bait the US into being distracted away from Europe, although the latter hinges on a telegram that could easily be a "retcon" after the fact to justify the blunder.)