@Phr4nk0: The game's mechanics are dramatically different than that of a typical first person shooter, and the game is designed to play by the tactics that work in paintball, specifically. Not the strafing, jumping, no-scope headshot kind of stuff you see in your regular shooter. The game rewards staying in cover because it's very easy to be eliminated. It has a highly controllable lean feature (called "snap") so that you can shoot opponents, while staying in cover. You can reload while firing, just like with a real paintball gun. Further, the game features slides and dives, things that would be great for any shooter, but are more key to the sport of paintball.
So the answer is, really, because paintball is fun for its own sake. And making a game that highlights the strengths of paintball is what makes it different, and viable, from any other shooter.
The fact that it's rated E10+ is a nice bonus. But lots of people that play paintball don't play it just because it's a simulation of war. They play it because they love the game of paintball. So making a game after it is just as reasonable as making a game of Chess, Tennis or Baseball.