“Neither the Supreme Court nor any federal circuit court of appeals has extended Commerce Clause powers to compel an individual to involuntarily enter the stream of commerce by purchasing a commodity in the private market. In doing so, enactment of the (individual mandate) exceeds the Commerce Clause powers vested in Congress.” from newsok.com
Here is the best explanation I have yet come across:
[T]he lack of a severability clause wouldn’t necessarily result in the overrule the rest of the legislation, which mostly have to do with spending and rationing — the expansion of Medicaid, Medicare cuts, and sweeping regulatory authority — and isn’t wrapped up in the mandate. This has been the Court’s approach to other issues, such as the recent Sarbanes-Oxley ruling, another law which lacked a severability clause, where they invalidated a portion of the law and allowed the rest to stand.
Some proponents of the ACA believe that the health of the individual mandate would move us closer to implementing a single payer system. (See Ezra Klein's article in today's Washington Post.) The danger of this is only too real.