People need some motivation to make these. How about all winning tilesets are used in a level? People aren't going to make tilesets just because they want to win something. If they don't get anything in return, it's pointless.
I've made tilesets just for the sake of making tilesets, and I think a few other people have too, but yeah, some sort of prize would be a good idea.
The problem is...what sort of prize? Getting your tileset used in a level isn't exactly validating; I don't think it would encourage people much. I suppose I could make some of those annoying signature banner things, you know, the ones that slap you in the face with all the "I WON THE TILESET COMPETITION AND YOU DIDN'T, NYAH NYAH, BUT UNFORTUNATELY ALL I GOT WAS THIS ANNOYING SIGNATURE BANNER," but...I dunno, it doesn't seem quite the right thing for this.
You can see my dilemma, I think.