Why just athletics?
well, with the exception of debate (and I suspect even a tournament championship debate doesn't fill a gym with spectators), those other activities are not competitive--a typical performance doesn't end with a "winner " and a "loser". our culture emphasizes and places a high value on winning. I'd guess the involvement of lots of adults adds to the hanky-panky quotient in sports, and hence the rules. I'd also guess the prohibited behaviors pre-dated the rules. not sure how many debate, drama, music and choir college scholarships are available, but their number is dwarfed by athletic scholarships. so there's money (or dreams of money) involved, which can bring out the worst in people.