I think Romney genuinely believes that the details of health care are better handled at a state level than at a federal level. A relatively affluent state with low unemployment, like Massachusetts under Romney, may be able to afford a health care policy that wouldn't be appropriate for a less affluent state like Texas or a high unemployment state like California.
There are also key differences between Obamacare and Romneycare, and especially Romneycare as proposed. For example, Romneycare did not restrict doctor owned hospitals from expanding. Romneycare left abortion coverage for individuals and small businesses up to the customer, instead of prohibiting it as Obamacare does through the Stupak amendment. I believe Romneycare permitted small businesses to band together into larger bargaining units, putting them on an even footing with large businesses; Obamacare prohibits that. Romneycare as proposed allowed employees to use employer health care benefits with whatever health care insurer the employee preferred, though that part didn't make it into Massachusetts law.
It's also to be noted that while Romney advocates repeal of Obamacare, he does also advocate replacing it with federal laws that would facilitate good state level programs like that in Massachusetts. In particular, he advocates the following at the federal level:
- giving health insurance premiums paid by individuals the same tax treatment provided to those paid by employers, important to allow individual choice about health care
- restricting insurers from dropping people from preexisting conditions if they keep continuous coverage
- block granting medicaid to states, allowing states to adopt systems like Massachusetts' without having to get special waivers for states for which that is appropriate
- restricting punitive medical malpractice damages, which distort the financial incentives on doctors away from providing the best possible care for their patients on a day to day basis
Romney lays out these positions here, if you're interested:http://www.mittromney.com/blogs/mitts-view/2012/03/why-id-repeal-obamacare