Well, it seems to me that paintball's rivals would be the other sports which take the most people away from it that would otherwise be playing. It has to be something fairly expensive, time consuming, and that people travel for to be in the same league. Golf, gambling, and sex seem to be about the best comparisons. Yes, sex is a sport. It involves skill, athleticism (If yours doesn't involve athleticism, I hope they don't waste their time with you a second round!), time and money expenditure, often wasted effort with no results, strategy, hellacious competition, years of training and experience, and with a bit of luck, cheerleaders.
At least that's what I can come up with as to what distracts people from paintball...