Rain usually happens because the hot air around the ocean carries water up above into the air and then falls into the land. Due to the rotation of the earth the prevailing wind mostly goes from east to west near the equator. Sometimes due to the mountains or due to the large stretches of land between the west coast and the east seas, rain is unable to reach the western coast. When the humid air from the east doesn't reach the west coast a coastal dessert forms. Even if there is some sea on the west, it evaporates and carries the rain to the west, not to the east.

This is a simplified explanation, since there are other factors to consider such as the temperature of the oceans, latitude, lakes, rivers, seasonality of the air currents or the direction of the prevailing winds.