A lot of small airports with very little traffic.

Counting airports doesn't give you an accurate picture, because just about any airport can have a GPS approach, since the whole point is that it doesn't require any ground equipment. That doesn't mean those approaches get much use, because people don't fly into those airports all that much, especially not on instrument approaches.

I fly out of an airport with a published GPS approach and no ILS, and people use the GPS approach on well under 1% of the total landings there. Any reasonably busy airport will have ILS.