The PS3 failed, and microsoft can't really improve on their system without copying the Wii, we hated Six Axis on the PS3 (even sony new this, why else would they release Dual Shock 3 controllers), and we would hate them for copying Nintendo. Bad move microsoft.
Besides, the demand that Xbox 360 kind of users want is just better, bigger, and more impressive games. Not any of this fancy motion sensing.