Throughout most of U. S. the nation's population center has continually moved toward the west. Why?

Because west was uninhabited by the Europeans, it was rich with resources and when the railway was created many people rushed to the West in search for gold.

Explanation:

Europeans firstly settled on the east coast, where they created first 13 colonies. As the time passed the population was growing fast and the natural resources were exploited. That is why there was a constant need to move towards the West, which led to most of the people settling new lands.