Quite simple really; Hitler couldn't finish off Britain, he had been defeated. Even with that, Hitler always had his eyes on Russia, and attacking later would have been devestating. Russia was re-equipping its military, and would have been ready for 1942 which would have caused major problems for Germany. Operation Barbarossa was set for May 1941, the Russians wouldn't have been ready, and completely surprised. They thought how you are now ('They wouldn't attack Russia until finishing with Britain'). The Germans saw this as a perfect oppurtunity, and they were right to do it then. Russia had 4.7 million troops in June 1941, and only 25% were on the frontline with serious delays getting the rest to the front because of poor road and railway.
It was a very smart time to do it, and if they had attacked in May 1941 I dare say they would have won, avoiding the Russian winter.