I may be off the mark here but during my ATPL ground school i was told that the Americans can turn off the GPS system at any time they so wish (for security reason etc). I believe that this happened during the Sept 11th hijackings. Another point is that the accuracy of the system can be manipulated, again by the Americans, so as to confuse the pilot as to where he is at the time, his speed and altitude etc. Again this would be for security reasons i presume.
Maybe the fact that the system may be turned off at any time without warning, or inaccuracies implemented into the system during an aircrafts final approach to land is the reason it has not already been implemented for use?
I await to be stood corrected!
Regards
The Foil
