Why are the mountains located at the west?

Assuming you mean the mountains of the western United States, there are two sources of their formation. The Rockies were formed when the Farallon Plate, a tectonic plate that once lay beneath the Pacific Ocean, collided with and slid under the North American Plate. As this happened it compressed the crust and dragged against the bottom,pushing mountains up. After the Farallon PLate was gone, the continent started to pull apart enough that bloacks of crust sand downward, leaving mountain ranges as high areas to form the basin and range province. The thinning of the crust also resulted in some volcanic activity.