The Appalachians have had 3 orogenic (mountain building) periods followed by significant erosion. What you see currently is the result of the last erosional period. Imagine something much closer to the Himalayas (similar continental convergence process) at their highest. Appalachian geology is rather complex compared to other mountain ranges. The Rockies aren't quite comparable since they formed due to eastward migration of a shallow angle subducting plate. Trust me, I'm a geologist.