Because Non gamers are not really interested in LBP, the whole concept of the game seems to be very confusing for the non-gamers, and it will not make them buy PS3 for it. Plus not all PS3 owners are interested in a game like this they prefure those "Mature" titles.
Wii Music has more appeal, wider userbase and more pick and play feeling to it. Therefore it will sell well, i'll buy a copy as well sooner or later even if i wont be a primary user of this game.