Historical reason for Germans in Austria?

Question

Why is Austria made up of almost entirely Ethnic Germans? What's the historical basis for this, and when does it date back to?

Answer 1

The German people had hundreds of small kingdoms and principalities in Europe. That was why Austria was in the Holy Roman Empire, but only the area we now call Austria. Other Kingdoms were Saxony, Bavaria and the other major power Prussia.
Austria had a huge nation state called Austro-Hungarian Empire. Prussia did not like the Austrian Empire. Later after Napoleon, the German States grew closer, but Prussia shut out Austria when it formed the North German Confederation. Later the southern countries joined creating the German Empire. Austria was cut out. The Kaiser was the empire over the Kings of Bavaria and the rest, save Austria.
Then in WWI the Austrians and Germans sided with each other and both lost. The Austrian Empire was destroyed, leaving only the German State called Austria. So since the middle ages, Austria has been ethnic Germans. Hope this helps.

Answer 2

Strictly speaking there is no historical reason at all, since history is the documented past. The Germanic presence along the Danube predates written records. If you want to know about the origins of the Germanic languages or the cultures associated with those tongues, I would suggest that this is a question for archeology and paleolinguistics.

To put it another way the Germanic presence predates Austria.