Because some stereotypes when extended to a large group of individuals can, for marketing reasons, be considered more or less true. "Young girls love fashion shows more than violence and sports" is one of them.contests aren't girly, and the pokeathon isn't jsut for guys. Seriously, why do people think so stereotypically?
Contests were only elevated to their current status for girl appeal, and even with that aside, Pokéathlon wasn't around when contests became a big deal. Considering how much emphasis they put on contests, I don't see them ever removing them unless they stop being part of the games.