This is a surprise? Have you played the game online lately? It used to be that whenever I got on, there would be at least one room in the World Mode that was 90+% occupied. Now, there have been times when I've gone in there and there was no one present in any room at all.
I think that's more that World Arena's novelty wore off. It's ok, but not really a good substitute for rank or player match (if you like voice chatting).
I guess the TTT2 community is different from the SCV one because Global Collisseum even now is populated. I personally like World Arena better than Ranked or Player match.