Because its costs were too high to attract participation, as reported here:
Monthly premiums would have ranged from $235 to $391, even as high as $3,000 under some scenarios, the administration said. At those prices, healthy people were unlikely to sign up.
Well duh! Healthy people who signed up at age 50 not long ago could get excellent coverage for two people for less than $60 a month through Barack Obama's favorite fascist, Jeff Immelt of GE.
Government does very little well, and never cheaper than the private sector.