It's because the Senate has never ratified the International Covenant on Economic, Social and Cultural Rights 1966 (ICESCR).
In 1948 the United Nations General Assembly ratified the Universal Declaration of Human Rights (UDHR).1 The problem was that declarations by the General Assembly are not binding on member nations. They amount to powerful suggestions and nothing more.2
Eventually, member nations wanted to make the UDHR international law. International law is establish by treaties and maybe to a lessor degree UN Security Council resolutions. Therefore, the UDHR was divided into two international covenants, the ICESCR and the International Covenant on Civil and Political Rights (ICCPR), that could be ratified by member nations and thereby become international law. According to the University of Minnesota Human Rights Library, the United States signed both documents on 5 October 1977. The Senate has only ratified the ICCPR. I would imagine one of the reasons the ICESCR has never been ratified is because of Article 12.
Auricle 12 of the ICESCR states:
1. The States Parties to the present Covenant recognize the right of everyone to the enjoyment of the highest attainable standard of physical and mental health.
2. The steps to be taken by the States Parties to the present Covenant to achieve the full realization of this right shall include those necessary for:
(a) The provision for the reduction of the stillbirth-rate and of infant mortality and for the healthy development of the child;
(b) The improvement of all aspects of environmental and industrial hygiene;
(c) The prevention, treatment and control of epidemic, endemic, occupational and other diseases;
(d) The creation of conditions which would assure to all medical service and medical attention in the event of sickness.
Americans are under the illusion that we are the champions of human right in the world. We might have woken up a bit since we learned we'd been torturing prisoners of war, but lots of us don't really count that. Funny thing is that most of the people who wanted John McCain for president partly because of his war-hero status, are the same people who don't have a problem with the fact that we've been torturing terrorists. We are a strange bunch.
For what it's worth the human-rights-trampling China has agreed as part of it's treaty with Great Britain, the Joint Declaration of the Government of the United Kingdom of Great Britain and North Ireland and the Government of the People's Republic of China on the Question of Hong Kong signed on 19 December 1984, to honor the ICESCR in Hong Kong. (Bilateral Treaties in Force as of January 1, 2009)
According to Anup Shah of NPR, "The idea of universal health insurance isn't the brainchild of the Obama or Clinton administrations. It started in 1912 when a presidential candidate running under the Bull Moose Party called for insurance for all workers. It was Teddy Roosevelt, trying to regain the presidency. Since then, steps have been made toward universal coverage (along with many other medical innovations), but each president, Democrat or Republican, has either hit roadblocks -- or created them."
There is an excellent time line on the Health care debate at http://www.npr.org/templates/story/story.php?storyId=111089777
For more information on Human Rights go to the Office of the High Commissioner for Human Rights website.