The Robber class of trainer is not commonly seen throughout the core series. When they do show up, however, they virtually only show up in one place, such as when they're found in Pokémon Mansion and only Pokémon Mansion in Gen I. Or the Goldenrod Underground and only the Goldenrod Underground in Gen II, and only as you're going through the Team Rocket section to rescue the director. To me, however, the most bizarre thing about them is not their scarcity, but their teams. In Gen 1, it was Fire-types and only Fire-types (I'm sorry if I sound like a broken record.); in Gen II, they did include some Team-Rocket-esque Pokemon to suit their personality better, but Fire-types remained a solid staple throughout your battles with the eccentric crooks.
tl;dr Why do Robbers like Fire-types so much?