It is not the united states responsibility to provide health care to countries other than our own. Even though it may be a good thing to do so, we cannot spread out our resources so thin. There are still areas within the US who need the help and coverage more so than another country.
In an ideal world, of course we would care for Mexico since it is so close to the US border. But we are having debates over providing certain benefits here. I do not think we can even consider offering health care to Mexicans in Mexico at this time without causing more political upheaval here.