Answer:
The strategy used and the conditions of war.
Step-by-step explanation:
When Germany invaded Poland WW-II hadn't officially began yet. The Nazi Germany had been just adding countries to expand their terriotories, it is when Poland is occupied that the UK declares war on germany. The occupation of Poland was also extremly fast and quick under the "BlietzKrieg" strategy which was design to strike fast using a combination of both aerial and ground forces.
Different from Poland, when the Germans intended to defeat the soviets and began their invasion, WW-II was already taking place, which created a scenario in which the german forces were split in different fronts having no chance of using again their BlietzKrieg warfare strategy as well as the rush Germans had on conquering the USSR before the winter had struck, which as it's well known ended up as an unsuccessful mission after all.