funny "Robocode has peaked" quote
Robocode is NP-complete as most games with depth. Other games might include chess, go and poker.
But Robocode is more than simply a complex game. Having tanks and bullets as theme makes it a lot more fun. There is an intuitive feel about how a tank should behave. Moving and aiming efficiently is simply not enough, robocoders like to make theirs bots move smoothly or turn their guns without shaking.
The engine being open to everyone, with anyone being able to develop a bot (not necessarily competitive) and also being able to upload them in an open internet environment removes most barriers to entry. And being an AI competition ensures there is always someone to compete against. A great deal of Robocode longevity can be credited to RoboRumble.