Why cant' mullatos in the u.s be called mullatos.?

mullatos like tyra banks and barrack obama be called mullatos not blacks.their are different lineage.for and example mestizo's which most people in latin america are call themselves mestizo's.they dont called themselves indians because most of them have dark skin and they dont call themselves spaniards they call themselves mestizo's.just a mixed group of people well why dontmullato's call themselves mullatos not blacks or whites just because they have that kind of complection.