Assuming we're talking about the North American continent (which makes sense), the simple answer is blame that huge spine of rocks running up and down the height of the western portion of the USA and Mexico called the Rocky Mountains and the Sierra Madre's.. Think of these two ranges as one giant range. It acts as both a giant wall and as a weather factory, and the colder weather created there moves from the spine of mountains and goes only one way - east. It goes all the way east until it gets to the Atlantic or Caribbean. .Anything west of this range is a giant coastal plain that is warmed by the waters of the Pacific ocean and El Nino current and that heat is trapped behind the opposite side of the wall. Any precipitation incurred here falls as rain and not snow, and in some areas precipitation almost never forms at all, and hence you have desert. If you want to see a visual depiction of this, a great source is the USDA Plant Hardiness Zone map, and specifically take a look at the west coast.
