A big reason was President Wilson and his policies at the treaty of Versailles. One of Wilson's fourteen points was the principle of national self determination. And that is basically just the idea that people should be able to choose their own national government in nation states. At the treaty, France's prime minister, George Clemenceau, wanted to actually create a new state in Germany, a Rhineland buffer state, but that idea was eventually rejected. Since Austria Hungary was composed of many pockets of different national ethnicities, it only made sense that most of them be given their own country, so the treaty split up the Austro-Hungarian empire based off the principle of national self determination. Germany was spared this because most of it's people were ethnically German. The Ottoman Empire did contain many different ethnicities as well. But they were not given their own countries after the war, besides the country of Hejaz. The main reason for this would be because the French and British had made the Sykes-Picot agreement of 1916, where they agreed that former Ottoman territories would be administered by France and Britain under what was later termed the mandate system.