A map tournament where the ideas are not implemented in the mapmaker is much more forum-friendly than one where you have to use the in-game mapmaker.
Not everybody knows the tricks of the trade, and not everybody can upload a playthrough from their console to the internet. The contest would really only be limited to a few users on the forum, and the ideas themselves would be limited by the mapmaker. As good as some of you are with the mapmaker, there is still a limit on what can come out the other end.
I agree that there should be some sort of mapmaker tournament for you lot, but anybody can create and submit a map idea drawn on paper or on a computer program for this. I'm surprised that people aren't interested in submitting their own map ideas though.