Why were Germany and austria not allowed to unite after ww1?

Answer 1

Since 1945 neither the Germans nor the Austrians have been at all keen on unification.

Answer 2

Germany was not unified in 1848-1850 because there were too many individual principalities. Each wanted things their own way, and none were willing to give up their power in order to unify.

Answer 3

because of Anchluss stated that it was forbidden. Britain and France feared that if they united it could lead to yet anther world war (and later it did but for other reasons too...)