There is no ELI5 answer to a question of this complexity. Seriously it happened 80 years ago and it is still an active topic for serious inquiry.

My top most opinion is Germany underestimated the Soviet union's ability and commitment to fighting.

Germany and The Soviet Union had a non-aggression treaty to divide Poland. If the Germans had abided by the agreement they would not have expended so much of their resources in the Eastern front and might have successfully controlled Western Europe.

https://www.britannica.com/event/German-Soviet-Nonaggression-Pact