Zombie Master, Zombie Panic: Source, Pirates, Vikings and Knights 2, Age of Chivalry and possibly even NMRiH. All these mods hit over 100 people almost everyday.
And two of those mods have ceased development. People would play TTT if it were a Source mod, but like Anon said, too much work and would require additional ressources like new models, guns, etc, and those who make them.
TTT as a game-mode in GMod attracts FAR more than 100 people a day, turning it into a sourcemod wouldn't attract anywhere near as many players. Source mods generally get attention when they're first released but slowly die due to being too obscure/lack of advertising.