Why are so many of the top WW2 fighter aces German?

Early in the war, the Germans had a technological and tactical advantage in the air in almost every theater. Up until 1942, Britain was the only combatant nation with front-line aircraft in enough numbers that were capable of engaging the principal German fighter type, the Messerschmidt Bf-109, on equal terms. The Polish PZL P.11 was obsolete in 1939. The French air force in 1939 was in the middle of a restructuring, and obsolete aircraft such as the Morane-Saulnier MS.406 were the principal French fighter aircraft while they struggled to put enough newer and satisfactory aircraft, such as the Dewoitine D.520, into service. The Soviet air force was also in the middle of a similar restructuring when Germany invaded in June 1941.

These factors, together with good pre-war training of German pilots, meant that between 1939-1942 German air forces aces racked up a tremendous number of kills, especially on the Eastern front. Also, while the Allied powers followed a policy of rotating pilots, where aces and experienced fliers were retired from combat operations. Adolph "Sailor" Malan, for example, one of the leading British pilots during the Battle of Britain, became a Wing Commander and was then retired from combat operations in mid-1941, and didn't fly many more sorties during the rest of the war. The same happened in the American air force. I don't know about the Soviet's, however. The Luftwaffe, and the Japanese air force, had no such rotation policy, and combat pilots often flew until they were unable to, either through death, capture, or injury. Pilots certainly had leaves and rest periods, but were always expected to return to the front lines, and the elite pilots were put in charge of their own fighter squadrons and expected to lead them in combat rather than being rotated back to Germany for the rest of the war.