Well, a lot of them have powers that would lend to their fighting skill, like Karnak's ability to see weakpoints and Wolverine's healing factor/indestructible skeleton. Psylocke in particular should perhaps be higher, being able to telepathically read her opponent's next move and telekineticlly enhance her own fighting abilities (one or the other in the past, but apparently he's got both these days).
It's interesting that between the two companies, DC has a higher proliferation of non-powered martial artists.