One thing you have to remember is that pre-WWI, the political leaders in Austria and Germany saw each other as rivals. There was no strong push for unification up to that point. Bismarck saw the Catholic Hapbsurgs, who ruled over a patchwork of loosely associated non-German peoples in eastern Europe, as a threat to Prussian dominance.

Pan-Germanism picked up after the war, spearheaded of course by Hitler and his radical vision for the unification of all Germany speaking peoples in Europe. One cannot overstate the significance of the link between the NSDAP and the issue of Pan-German political unification: it was basically the Nazis' #1 trademark issue from the beginning. Hence the entire Pan-Germanism movement was subsumed in the NSDAP.

Of course, at the end of the war, the Allies disbanded the NSDAP and the Wehrmacht. Millions of ethnic Germans whose families had been living in Eastern Europe for centuries were forcibly relocated west of the Oder–Neisse. At this point, it was in Austria's best interest to distance itself from Germany as much as possible, and paint themselves as Hitler's first victim rather than his accomplice. Pan-German unification would not have been a political tractable position for political leaders in either country. In West Germany, political power deferred to the two existing institutions that had held the most power in pre-war Germany: the Church (which powered the formation of the CDU), and the Socialist Party (i.e. SPD). Kurt Schumacher, the leader of the SPD, was somewhat interested in uniting East and West Germany under a socialist (but not necessarily Soviet-dominated) regime. On the other hand, Konrad Adenauer, arguably modern Germany's "founding father", had absolutely no desire to incorporate the Soviet controlled East. Up until that time, Germany had never really seen itself as part of "the West". Adenauer made it his #1 goal to change that. Therefore he abandoned any notion of a partnership with the East and fostered a deep relationship with the NATO powers.

At that point, the idea of even an East-West reunification was politically intractable until the end of the Cold War.

Edit: fixed acronyms