Epel has a very special relationship with his Pokémon. In a town with very little youngins to play with, he's learn to appreciate his Pokémon more! Of course he loves the idea of having a traditional, macho-looking Pokémon, but he has a real soft spot for Applin. So much so that he refuses to evolve it, and now it's grown so big!
He could evolve it, but he doesn't need it to be a big dragon to know it can hold its own in battle. Epel raised it, and that's proof enough of what Applin is capable of!
I think this may be my favorite dynamic in the series so far 🥺