That is a simple question, but the answer is very complex. On the bottom line, I would say because it was build on a foundation that was not sound, by people who were lunatics. It was only made possible out of the desperate situation of the Germans during the Weimar' Republic between 1918 and 1933. This enabled Hitler to come to power and install his regime based on semi mystical ideals and race ideology, aimed to revive a new version of the Holy Roman Empire of German Nations, as it existed in the medieval time and ruled most of Europe. But in the political situation of Europe in the early 20th century that was delusional. After being at war with just about the rest of the world, the fall of Third Reich was inevitable.
Mainly because Hitler's ego made him bite off more than he could chew. Horrible policies and dwindling manpower and it was sure to collapse. Might have been different if he would have listened to his Generals and stepped aside and let them do their job. Hate will never win a war. If Hitler would have been assassinated , things might have ended differently.