If I understand your question correctly, you know what a commonwealth is. You are asking why the state is called the Commonwealth of Massachusetts. I think you will have to go back in history to when the state was named. Remember that the first settlers lived in Plymouth Colony and Massachusetts Bay Colony. These colonies were joined into the Commonwealth of Massachusetts for the common wealth or common good of all of its residents, English and Indian. Generally speaking the same thing happened in the other "Commonwealth" states. Georgia and New York were established as single large colonies. There was no merging of colonies involved.