Why have so many spanish words become part of the american englis?

This has happened because people who's first language is Spanish, have moved into the U.S. and since at first not many of them new English over a period of time some of the Spanish words became a part of English, and some of the English became a part of the Spanish.
The mixture of the languages is common when 2 or more are mixed in one area, so everyone tries to understand each other by putting another language within their own when speaking.
                                               I hope this helps :)