Why was the Eastern Front the main front in the Great Patriotic War?

Because the largest forces of the USSR and Germany were concentrated in this sector. In fact, the fate of the eastern front determined the outcome of the entire Second World War. With a very high probability, if Germany could defeat the USSR, the war would be lost by the allied states, that is, Britain and the United States. One way or another, but the military machine of the USSR was able to defeat the German troops.