The main cause of cultural diversity that I see resulted from two things: (1) slaves that were brought to the united states against their will to work the plantations, as they still do today, and (2) immigrants fleeing from their countries for a better life. All of these cultures just interacted, slept together and we are quite diverse.
Do you mean what is the AFFECT of cultural diversity? The cause of cultural diversity is the coming together of people from many different ethnic and religious backgrounds, such as the immigrants who helped establish the United States.