But, you're asking why. I guess I could say that, given the Dark type's concept, it's far easier to design an elite 4 member than a gym leader for it.
An Elite 4 member generally can have a pretty exotic background and past, and stay shrouded in mystery. For their characters what matters is their philosophy, it's their central focus, and how they reached this philosophy is left for the player to guess. This makes the Dark type extremely fitting for an Elite 4.
On the opposite, a Gym Leader is a more open person. Someone people look up to and that their hobbies or job help society. The focus of their character is what they do, how they act. It's certainly nowhere impossible to make such a figure with a Dark type motif, but I'll give them that it's harder than say, a Water or Rock specialist.