For the same reason that we should not have the goverment direct our career and educational paths, compele us to accept a goverment chosen mate, etc. So long as universal coverage is an extra-constitutional mandate, compulsory universal coverage is an unwarranted intrusion.
My alternative: amend the constitution so that the goverment becomes responsible for ensuring that all Americans have no-cost health care available, financed by a seperate tax, AND so that Americans that choose not to suffer the inevitable additional wait times may choose to seek care from alternative sources at their own expense AND so that citizens may opt out of universal coverage at their own peril.
Or
The significant majority of un(under) insured Americans that are so by choice may simply choose to start paying their own way, and the small remainder of those that are not un(under) insured by choice could be covered by the goverment.