TLDR: They stretched themselves out too far, too fast, and fought too many enemies at the same time. They ran out of raw materials, and lost all of their best leaders.

The Nazi's needed to maintain a technological superiority to overcome their lack of natural resources. By the end of the war the allies had mostly caught up and had the industrial might of the USA backing them up and supplying them.

After some early successes in Europe they became over confident and began attacking on too many fronts at once.

Once the Soviet Union got involved the Nazi's couldn't compete with the massive weight of bodies and industry the Soviets could throw at them. While the quality of the Soviet Unions troops and equipment was very poor their sheer numbers overcame their disadvantages. The Nazi's highly trained personnel couldn't be replaced quickly enough and their advanced weapons could no longer be made in adequate quantities.

The Nazi's were also running out of key raw materials like metal and Oil. So even if they had the planes and tanks, they couldn't fuel them.

By the end of War Hitler had replaced all his competent generals with people loyal to him (yes men) and he had taken personal control over many military decisions. Hitler became deluded and was being affected by his addiction to various medications. He made bad decision after bad decision, and several failed attempts to kill him arguably made the Allies job easier.

The loss of the Battle of Britain is cited as the beginning of the end. If Britain had been captured, or invaded it's far less likely that the US would have stepped in to help them directly and instead would have focused 100% on the Far East.

That and the Italians were idiots. Some historians argue that the Italian military was so inept that they consider them to have been on the side of the allies.