The gym leaders in DP have the Rock, Grass, Fighting, Water, Ghost, Steel, Ice, and Electric type specialties. The other Elite 4 members specialize in Bug, Ground, and Psychic typings. This only leaves Normal, Flying, Fire, Dragon, Dark, and Poison types. We can immediately rule out Dragon type because Cynthia uses Garchomp and the others are Legendary Pokemon. Not many of the Normal types are that strong, and most of then can easily be found and obtained. Additionally, some key trainers already use some Normal types. A lot of Key trainers around use the Dark type options, so there wouldn't be anything new. Poison types are used among key trainers as well. As for Flying, a lot of them aren't that strong, can easily be obtained, or are used by key trainers. A Flying type gym would probably be a much better option than a Elite 4 specialty. The Chimchar line can only be obtained at the start of the game and aren't used by any key trainers. Ponyta's line takes quite a bit of grinding to evolve. Both of those Pokemon aren't weak at all. The other Pokemon aren't fire types but are obviously for type diversity and have Fire type moves. Another reason could be that these Pokemon fit Flint's personality. So while there aren't a lot of Fire type options, the ones that are available are strong. (Key trainers are Gym Leaders, Evil team Leaders and Admins, Rivals, Elite 4, and Champion.)
This is based on my analysis. Hoped I was able to help.
