there are a few reasons for this:
-most people cannot notice a huge difference between 60hz and 120hz
-the only way a pc can output 120hz is dual dvi, 60hz is the standard for displays.
-some people run two monitors so are limited by connection ports
-120hz monitors are TN panels which are horrible compared to other types
-most computers can not get 120fps on new games. for brand new games this number is often less than 60fps
i'll take a H/P/S-IPS panel @ 60hz over a TN panel @ 120hz any day of the week. viewing angles are over 10x better, color accuracy is there, response times are higher but for anything under 16ms maximum there is no ghosting. such panels can not dispay the A-B-A-B 3d frame sequencing but can do a half resolution 3d interlace if required.
there are 120hz computer monitors out there, i just can never find myself recommending them unless you want the 3d swapframe capability.