Supply and Demand. As long as they can find migrant workers to work for minimum wage there's no urgency to raise wages to attract US Citizens.
and its the same reason why we don't have agricultural investments in automated harvesting of vegetables - why bother when the migrant farm workers work for next to nothing. maybe someone could invent a machine that cracked crabs and harvested the meat, but we will never know because no one invests to invent one. its the same thing in industries going to china - why can't we have US based automated assembly of furniture for example? at least we would provide some jobs here in the US in the industries needed to support the automated machinery, instead of sending it all to china.