Terrain and obstacles are a possibility but IMO this could/would be in a minority of approaches at certain airport and when you look at what is capable with some GNSS approaches like the one in Rifle it is hard to believe some sort of GPS approach couls not be made available to most runways.
Those are RNP AR approaches, which require a very high equipage bar. Most airframes simply do not have the equipment.
Besides if with much higher minimums a circling approach to a runway is available I'm pretty sure a GPS approach to that runway could be designed.
Alas, that is simply not the case.