The correct answer is - because the US is a mix of many cultures and languages.
Because the United States are a country where there's people from all over the world, which brought hundreds different cultures and languages with them, the music has evolved in a specific unique manner. In general, we can see in the United States's popular music the elements of the Celtic music which is mostly into the pop music, the country music that evolved as separate traditional music from the people living and working on farms, the Latino elements from the big Latino population, as well as the music types originated from the black population like the blues and jazz that were inspired by the hard life and slavery.