Geographic reasons why the western half of the united states more sparsely populated than the eastern half

People tend to live on land that is reasonably flat and where crops can be easily grown. 
What kind of landforms do we find in the west?   

While winters are rainy, summers are dry. regions with this climate are usually in the eastern part of the continents. they experience hot, humid summers and colder winters. precipitations have a regular distribution throughout the year, but hurricanes are common.