Because it was as useful as 3D on the Nintendo 3DS, motion control on the Wii, Gamepad functionality on the Wii U, the Move controller on the PS3 and Kinect on the Xbone/360.
Basically...useless tacked on crap from hardware manufacturers to be 'different' just for the sake of being different. --- I was modded for posting a link to GameStop requiring fingerprints for all trade-ins. Reason given: Off Topic The Mods here are the worst I've ever seen. Ever.