There were multiple problems with setting up and running TNT. They always ran into problems setting matches up and had a bunch of downtime between them. They always lost track of how had already gotten into some matches and ended up having the same players throughout the show. It mostly boiled down to them playing whatever the newest multiplayer game (usually an FPS) was at the time, which seemed like they were endorsing them. Then add all of the problems the crew seems to have when they try to get consoles online in the new office. I can easily see why they haven't brought TNT back.
For the most part, it seems like an inefficient use of their time as well. There're plenty of segments in UPF that the community can and does get involved with, and there're more chances to show off more games in UPF and Quick Look Live (compared to spending 2 hours on a single game that may or may not be worth that much time showing off).
I do think they should do more of the specialty shows though, like Load Our Last Save and Breaking Brad. Anything that could result in something like "Backflips in Bioforge" should stick around.