Why did Germany have to take the responsibility of itself and its allies for causing all damage in World War 1 and Austria-Hungary not take an equal responsibility?

Answer 1

Because Germany was the biggest threat to the Allies during the war, and thus the Allies wanted to weaken Germany so as to not have to face them again in the near future. Also, Hungary's territory was reduced by 70%. As every nation was essentially drawn into the war by various treaties, this was completely unfair (and would later be a large factor for the popularity and support of the Nazis, who denounced the Treaty of Versailles).