Honestly? I don't know - they just don't want it in...
I imagine the major factor of it is that non of the guns have recoil (Looking at this from a marine aspect initially) - No recoil means any one with a good aim can hit and continue to hit exactly where they want. CS has location damage because it is a significant skill to continue to shoot the same spot - you have to control the recoil... isn't an issue in NS.
There is a trend among a lot of FPS - the 'Sci-Fi' futuristic style (more deathmatch orientated; eg quake3, the Unreal series (excluding the newest) tend not to have location specific, but the more realistic style FPS (slower pace, eg; dod, cs, etc) do..
A broad generalisation I know but it works