kinda curious why Germany didn't allow the plane to land
Not sure if you are being sarcastic or not, but just in case:
There were massive tensions between the German government and Erdogan recently, starting with Erdogan parodies and (kinda vile and personal) attacks on him being broadcast on public television channels, leading to a bunch of exchanges between both governments concerning freedom of parody/speech discussions, that then happened within Germany as well (some people got fired), up to the point where Erdogan refused several German government officials the entry to visit German stationed troops in Turkey.
The relation is currently strained, to put it optimistically.