Under the Treaty of Versailles, Austria was forbidden to join Germany. By 1938, with Austria merged with Germany under the Anschluss by Adolf Hitler, Austria was part of Germany. What reason made political union of Austria with Germany impossible?
Could you clarify your question slightly? Do you mean why was Austria subsumed into Germany in 1938 rather than forming an equal Germany-Austria? Or why was it banned from joining Germany in the Treaty of Versailles? Or are you asking why all the German-speaking states united to form the German Empire in 1871 except for Austria which was frozen out?