Blasphemy against the Holy Spirit?
That, and the fact that's it's a F2P FPS that even a massively outdated computer could run. It's no wonder you see so many Mexican, Brazilian, Russian and Polish children on TF2 matchmaking.It mostly still has a playerbase because they want to stick it to the people who have been calling TF2 dead for years. It's pretty interesting.
TF2 probably won't die until Valve officially kills it off themselves.