The United States does not participate in the Commonwealth Games because it is not one of the 53 members of the Commonwealth of Nations. The Commonwealth of Nations are the nations, states, and territories that are ruled and are noted as being under the British monarchy and throne. While the United States was founded and ruled at one time by the British throne, they no longer are and thus are ineligible to be able to participate in the Commonwealth Games.
The Commonwealth Games take place every four years and have done so ever four years since 1930. The origins of the Commonwealth Games actually can trace their origins and inspirations to the 1911 London Inter Empire Games. The Commonwealth Games are patterned after the Modern Olympic Games which also takes place every four years.
A very interesting fact pertains to the 2018 Commonwealth Games. This rendition will become the first major international sporting and olympic style event in which both the men and women will be awarded the exact same amount of medals.