For a couple of reasons: - Special glasses, which sit weird when you wear glasses already like I do. - No real compensation for when you aren't sitting inside a narrow arc in front of the display. More drastic in the case of displays like the one pictured (and in the 3DS) but still a problem. - Tradeoff of image quality vs. 3D. Since you're effectively needing double the framerate, this tends to necessitate cutting down on scene complexity (games) or resolution (video) to pump up the framerate. This seems to be going by the wayside as computing power creeps up, but it'll strain your system if you want to run at anything higher than 720p for some things. - New hardware, such as a 120(or 240) Hz display which I have no interest in buying while my current stuff works.