The settlement of North America began in the East. Many people arrived from Europe and Africa and colonized this section of the continent. This was also the area where most cities developed, and where most people lived. As few people crossed the Mississippi, and fewer still ventured to the west coast, this area remained underdeveloped.
Few people choose to settle in the Rocky Mountain area because this area was not as well-communicated as those in the East. Moreover, it had no cities and very few towns. People who travelled to the region faced many difficulties, and could rarely travel back. All of this meant that not many people settled in the West until the construction of the transcontinental railroad.