Here is the issue and I'm 99% sure it's why there is an entire ban and it won't be lifted....
If they were to allow drones in certain areas people would be "confused" and fly their drones in other areas. I'm sure some would be "confused" and fly in very populated areas of the park. Trust me... it _would_ happen and it would happen often. Sure, they could start issuing citations but you'd still have "confused" people bothering others with their drones and citations are not going to help.
You don't think it would be an issue? Feel free to look back at a post I made of person flying right next to Delicate Arch in Arches National Park. Look at some of the responses from people right here in this form. They felt that they should be able to fly anywhere they wanted. They even took offense to my public service video, stating it was actually part of the problem.
So, many people would see that drones were allowed in National Parks and either not know where the boundries were or not awknowledge the boundies at all. We know that the NPS is stating that National Park attendance is at all time highs and that they don't have the man power to do there job so I don't see them looking to make their jobs more difficult.
I know first hand that there are thousands of acres managed by the NPS where there is not a sole for hundreds of miles. Some people think it should be okay to fly there. Well, if there is no one around, what are the chances that a ranger would be in that area and know that you are flying at a given time?
Personally, I would not fly in any area of a National Park but I have accidently flown in an area managed by the NPS (feel free to look at Hite, UT). In that case it's not a National Park and is about 70 miles from anything. I had no idea that the area was managed by the NPS so it fell under this restriction. I _should_ have known.