Taller mountains, like the Rockies, are younger: they haven't had the time to be worn away. The Appalachians are older mountains, run up far earlier, and they've had the time to be worn down through erosion throughout the years.
Apparently, the Appalachians were part of Pangaea
about 300 million years ago.
The Rockies were raised about 200 to 100 million years ago
. Of course, they started as an underwater range of little islands, which is pretty cool, too.
But really, think about the character of the two ranges: which one seems young and wild, and which one seems older and sleepier?
And, of course, how are they portrayed in our national consciousness? The Appalachians have a place of mystery, and things
happen there (think of where the Devil himself hangs out: Georgia, New England, the Carolinas). The Rockies are places where beer is brewed, where adventurous rock climbers and outdoorspersons go. The Rockies are places to be conquered, still, and the Appalachians are places for picnics, families, and generally safe hiking.