alfredofroylan on 09 May 2010
Looks like they're afraid of getting lost.
“As we see it, one reason why a number of people who love 2D Mario do not want to play 3D Mario appears to be because they are afraid to be lost in the 3D world by not knowing the exact directions, while they feel that they can play with 2D Mario with no such issues. One of the development themes of the original Super Mario Galaxy was to create a 3D world where people may not be easily lost, and the spherical shape was adopted as the game play theme for this reason." - Satoru Iwata
It's all about motion sickness
