I detest the ACA but this is factually incorrect.
The U.S. Supreme Court did not uphold the ACA in National Federation of Independent Business v. Sebelius in its 2012 ruling. What it did was uphold the individual mandate as a legitimate exercise of Congress's power to impose taxes.
The ACA is far more vulnerable to constitutional challenges on the basis of its usurpation of state regulatory authority over the insurance industry. The only problem is that such a legal challenge would have to be initiated by: (1) state governments, and/or (2) insurance companies. But as far as I know, none of them have filed any such challenges. WHY NOT?
As to why no challenges, the only thing I can offer is anecdotal: Here in the People's Republic of New York we have this horrible firearms law called the Safe Act which is obviously unconstitutional, but remains in place five years later with no court challenges that anyone knows of. Meanwhile, the second something contrary to any liberal policy gets passed, there is an immediate court challenge and the law is suspended. Again this is anecdotal, but that's how it seems to me and what comes to mind when you ask why no court challenges to Obamacare.