Probably because the federal government does not have the legal wherewithal to do so. By the Constitution, those rights belong to the states. Much like abortion rights, nursing/medical licensure, tort reform, etc
I'm curious about DNR's and why the legality varies so much from state to state? Why hasn't there been some initiative to simplify this and adopt a standard of care nationally?
There are some attempts to make somewhat of a standard but the individual laws vary state to state. SOme states have assisted suicide/compassionate dying. It is not the federal govenments job to mandate health care to the states hence the lawsuits now about Obama care being unconstitutional. Attempts have failed, or haven't even been attempted, because there isn't anything simple about these decisions.