This question was actually asked fairly recently and answered by /u/BigBearSD although it was posed differently. Czechoslovakia did end up with a tiny part of Prussian Silesia, called Hlučínsko, which had a Czech-speaking majority, but the rest of it was overwhelmingly German and Polish-speaking, and attaching it to Czechoslovakia would have been a completely blatant violation of the right to self-determination that could not have been justified by the need to maintain geographic integrity or long-standing historical borders. It wasn't in the interest of either the Great Powers or of the Czechoslovak representatives, whose whole thing was that they were an enlightened modern, democratic nation, not feudal militarists like the Germans and the Austrians.