Game Freak didn't want Archeops, a Rock-type Pokémon, to benefit from something on the behalf of being a Rock-type Pokémon so they removed ALL the gems barring the Normal gem instead of just the Flying Gem and Rock Gem so the community wouldn't suspect there was a bias against Rock-type Pokémon.
Game Freak ended up exposing themselves with Aurorus not being able to learn Power Gem like the motherfuckers they are.
