I actually wondered why people become monotype trainers in-universe despite the potential drawbacks. It became an interesting discussion on the Pokemon Tabletop Adventures Discord. People like to assume that a monotype team could be easily swept by a single Pokemon of a certain type (and some can like Chairman Rose,) but having a monotype team can be legitimately effective. Knowing a type well means being able to counter its common weaknesses. And on poorly-planned "diverse" teams, if the sole Pokemon capable of dealing with that type goes down, the fight gets a lot harder. Thus, it also forces non type specialists to up their game to up their game by considering other ways to deal with type matchups than just "If a water-type Pokemon appears, send out an Electric-type"... because said Water-type could be a bulky tank with Earthquake. Or they use a stalling strategy pairing Aqua Ring and Leftovers with one or more status effects.

Also, some types benefit from certain weather or Terrain effects. A Water specialist, for example, could easily create a weather-based team using Drizzle or Rain Dance. Or an Electric or Flying specialist can pair it with Thunder or Hurricane.

I guess that while there are many different was a person could theme a trainer's team, types have the most established... well... identity in-universe. There are expectations and stereotypes associated with users of certain types, after all. (There are non type specialists with heavy motifs, like Lucy from the Battle Pike and her serpentine Pokemon. But people don't talk about those as much in-universe.) Focusing on a type could mean being a part of a community of people who love that type.

A former Team Rocket grunt that took a special liking to the Poison type due to the sheer variety of poison pokemon they got saddled with during the Kanto operations and decide to work on ridding Kanto of leftover stigma against the typing after the Rockets disbanded.

Team Rocket's hardly the only group in Kanto associated with the type. You also have Koga's ninja clan.

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