It's one thing I can never find a good answer to. It seems like if German-Austria had been allowed to join the German Republic it would've take a whole lot of steam from the Nazis. It was the goal of most Pan-germanists that weren't Nazis as well. Also seems to me it would've been a good idea to make the German Reich a constitutional monarchy instead of a republic, but that's a whole nother can of worms. And I know they only did it because of commies, but thats besides the point.
Immediately after WW1, almost every Austro-German wanted to join Germany. Why did anyone think it was a good idea to prevent it?
I know that not everyone was down for it when the anschluss actually happened near WW2 and it's pretty controversial, and mentioning it nowadays is pretty taboo, but right after WW1 it was really popular, and would've saved a lot of people a lot of stress.
Why the heck weren't the Germans allowed to unite? Poor sods.
To further add to /u/DuxBelisarius great answer:
One of the main reasons, why this was not allowed was that it would have added to the German capacity to wage a war in the future.
Also, that the idea was popular in the first place for a short period in 1919 had a lot to do with the at the time ruling Social Democratic parties in Austria and Germany thinking that a reunification would strengthen them politically vis a vis their conservative counterparts. For conservatives in Austria as well as Germany, the idea of incorporating a mostly catholic territory resp. being incorporated into a strongly protestantic country didn't hold great appeal beyond a short time frame in 1918/19.
Also seems to me it would've been a good idea to make the German Reich a constitutional monarchy instead of a republic
But that was not what the people wanted in 1918. If you look at how the German republic came about, the deposition of the monarchy was seen as a necessity for ending the war and ending suffering. The bad rep the Weimar Republic gets has to do with its end but in the context of the time, it was largely welcomed.
And I know they only did it because of commies, but thats besides the point.
Then you know wrong.
Why the heck weren't the Germans allowed to unite?
Even back at the time, there were people who argued (as their are people arguing today) that Austrians might speak German but are a distinct culture with its own history and pretty huge differences to the Germans.
Just seems like the treaties of Saint German and Versailles were idiotic. They tried to punish the Germans too hard but at the same time didn't go hard enough. Just pissed them off without actually weakening them significantly.