The borders of Bohemia had been the same for centuries. When Czecho-Slovakia was created after WWI, they kept the historic boundaries, although some of the border districts had a majority of German-speakers. The Sudetenland had belonged to Austria before, not to Germany (unless you want to count the medieval Holy Roman Empire as Germany).
Hitler was just looking for pretexts--if there hadn't been a German-speaking minority in Bohemia, he'd have found some other reason. The border districts are mountainous, so until the Czechs were forced to give them up, they could have made a German invasion very costly.
The French were also involved in the Munich conference.
