Mostly tradition, but the main point behind that tradition might be that wrapless ensures a more "intimate" connection with the cue. I.e. more chance to feel the "resonance" of the hit, and probably even more importantly, a good grip. Wrapless tends to "stick" or bond to skin better than a wrap does, which means the player doesn't have to actually "grip" the grip area; the "tack" of wrapless allows a relaxed grip.
IMHO, anyway,
-Sean