For a while after World War II the Soviets allowed monarchy to remain in place in Bulgaria and Romania but this was tolerated only in the very short run during the transition to "peoples' democracy." It was just not ideologically acceptable to the Communists. Similarly, I doubt that the PRG in Grenada would have kept the residual connection with the British crown indefinitely if they had succeeded in consolidating power. In any event, in Russia not only the Bolsheviks but all the other socialist parties in the soviets were anti-monarchist, and a monarchist Soviet Union was simply not possible.