Well Don Wood can certainly speak about the genuine aviation standpoint much better, but even from a hiking standpoing, a GPS unit is EXPENSIVE. To get one that has no map capability, just for elevation, lat, and longitude can cost 100-300$ state-side. To get one with map capabilities, that's in the 5-600$ range, and that makes it only capable of displaying maps. You have to purchase each region separately, and usually the regions are relatively small. I can only imagine the cost of an advanced flight GPS if the personal ones can go so high!
VORs are universal, and accessible to everyone in any type of craft regardless of budget. There's no "do you have the right model? the right memory chip? the right version of ther garmin. etc. etc." Through VORs you are pretty much garunteed to be able to understand the instructions ATC is giving you, or has laid out in charts. Also, say it's a cloudy day, there's no searching for enough sattelites to triangulate a position, it's already being reported to you.
And if you wonder why airlines don't use GPS systems instead of their FMC's, the simple answer is that the FMC's do WAY more than a GPS ever could =).