The same reason I don't own a wii wheel, wii zaper, or any other attempt from Nintendo to make a cheap buck out of a gimmick accessory item. I can't find anywhere on Nintendo's site stating that the adapter is any better than wireless, and can't find any concrete testimonial elsewhere.
Judging from the utter half-assery that is Smash Bros online, i'd say Nintendo's technical end just sucks. These are the days of cable speed connections where 30 people play a FPS that tracks mouse movement, jumps, motion, physics, etc with zero lag. Two playing Smash Bros with lag is *******ery on someone's end at the developer, not ours.