@REI-JohnJoff the top of my head, I'd say it's a pure geological situation. Not sure about plate uplifts, but the orogony which took place left the mountain ranges going north south, bad places to build cities, good places for forests. Looks like the higher the elevation, the less population also. And...that's where we find the most public lands, where we can design long trails.
Another factor, our population is not at all dense, like, say, Switzerland, we still have plenty of room. flat, wide open spaces, to expand.
And, it seems to me, that our main expansion is more likely to occur in population centers, i.e. cities.
That said, there's massive - slow- encroachment into previously vacant areas, thank goodness for national forests, parks, wilderness areas, national monuments....America's Best Idea!
Off, course, this is all off the top of my head.