Germany was ethnically and religiously more unified. Germans were generally Catholic, Protestant, or Jewish and all of these people spoke German as their primary language, or Yiddish, which is partly based in German (and whose speakers were generally bilingual). This population thought of itself as German, though much of it in some parts were Germanized Slavs or Balts. Politically, they also in general saw themselves as heirs of the Holy Roman Empire, which was politically dominated by Germans during its history.

The biggest area where this was not true was in the Polish Corridor, which was carved out to give a resuscitated Poland access to the sea, including many who identified themselves as Polish.

Germany's impetus toward unity in later years sewed East & West Germany back into one nation.

That unity was never natural to Austria-Hungary. Austria-Hungary was radically different. It included Catholics, Protestants, Jews, and Muslims, Eastern Christian rites, and a huge assortment of ethnic groups.

The Hungarians and other groups hankered for their own nation; Magyars saw themselves as a distinct group, as did the others. Ethnically, Austria-Hungary included Czechs, Magyars, Germans, Croats, Romanians, Serbs, Slovaks, Slovenes, Jews, Poles, Ukrainians, Bosniaks, etc.

It was just too diverse, in the face of nationalism, to stay together as one entity after the monarch fell from power. Not even Yugoslavia, which included some of its lands, was able to last after the death of Tito.