Also Because . . of cost / performance ratio!!
As much as I can see the advantages of GPS - at least in theory and running under ideal conditions - as much I know also about the constraints using it for approach purposes. Too many possible errors and deteriorations are imminent. It takes a very serious approach to the topographical situation and the obstacles to even have the base at hand for a good calculation of the CRM - collision risk model. And even then a thorough calibration session of the procedure follows before it becomes published. Might look at first glance a bit too excited and bureaucratic, but the results count, not the wishes!
I fully agree with his dudeness the local situation will not allow for a really useable / mindful GPS approach. Even if invented and approved by the responsible administration the minima will be so high that a diversion to a better equipped and situated airport is the better solution.
Finally the ongoing trial with augmented GPS - GBAS -EGNOS - WSAS have taken several years of time and still remain in progressive development. It might become the replacement of ILS in the future, but even then as mentioned before -the cost / performance ratio has to be okay.
Jo