LogFAQs > #961821885

I think the issue is each region has a limited pokedex. Like you could have a dog theme trainer but there typically isn't enough in each region to give them more than 2 maybe 3 because gym leaders tend to have fairly common pokemon of their type that you have "normally" already had an opportunity to see wild.

In order for it to change I think Pokemon would have to decide to step their game up in terms of difficulty.