"The Appalachians are slump-shouldered and low by alpine standards, dwarfed by the Rockies, mere hills next to the raw and knife-edged heights of the Andes or Alaska Range ... they've dwindled to their present size simply because they've had the time to ..." -- Earl Swift, The Tangierman's Lament.
In Europe and in South America, mountains divide. Cross a mountain in South America and although you might still hear the language of the conquistadors, you'll likely be in a different country. Cross a mountain range in Europe and you'll likely have not only a new country beneath your feet but a completely different language ringing in your ears.
Why not in what we call the United States of America? How did it happen that one nation and one language stretches from the Olympics of Washington State to the Appalachian foothills of Georgia? Was it the speed and intensity of settlement? Was it because it took place during a technological revolution, first with trains and then automobiles breaking down barriers of distance and difficulty?