Because unaccountable bureaucrats who are nearly impossible to fire aren't running it, and the patient gets to choose his provider and shop price.  The patient also has a financial incentive not to over-utilize the services.  Providers serving HSA patients have to please them because they have other alternatives.  With a Single Payer Plan, the providers are the only option you have, and if they don't care what you think and give you shitty service, that's your tough luck.  (Think Department of Motor Vehicles.)

An HSA, if not over-used, can become a government-funded retirement plan if you stay relatively healthy.  It works just like an IRA.  And there is NO PAPER WORK with an HSA.  You buy your health care just like you buy your groceries.