This animation does a good job of explaining why the Appalachian Mountains, and the Scandinavian Mountains, are so much softer than, say, the American Cordillera (which includes the Andes, the Sierras, and even that mountainous peninsula on Antarctica).

The “Taconic Orogeny”, which happens basically at 0:04 seconds (400 million years ago), forms the mountains that later become the Appalachian and Scandinavian Mountains, as well as the Atlas Mountains in Morocco, whereas the Cordilleran range only forms at about 0:20 second into the animation (closer to 225 million years ago). Interestingly, the Alps only form at a time almost indistinguishable from the present. Those are some seriously young mountains.