I used to be a full time knifer, but after playing some other games and servers, I just couldn't play a knife tdm for more than 5 minutes before leaving. The problem is the idea is simple, and just can't stand alone on its own. Knife TDM/DM's are simple: Run around on floating platforms and kill other people until you win. Over and over.
Once you get the same feeling of satisfaction, then it will start to become stale. Just go play another game for a while and then try to play it.