3 reasons I can think of
No serato HID support yet so if they use serato they would rather the MK1
THe Nexus decks when released froze a lot, and yes, even with a Pioneer DJ at their own booth but those issues have been resolved so the dj's not wanting them may not know
Some people are more quick on the MK1 because the menus are slower going from playlists to tracks and it forces you to have the wave moving during a track making you always have to hit back to get to your playlists, i know some dj's that hate that