Perhaps the most head-scratching aspect of the Republican presidential debates has been that no one has been able to land a hard blow on Mitt Romney’s record on health care. But there are several good reasons why it hasn’t happened. These reasons may turn out to be far more consequential than anyone would have thought.

5. Romney has been running for president since 2007. As many others have remarked, Romney has been refining his positions for many years. He is clearly a much stronger candidate this time around than he was in 2008. But there are plenty of other people who run for President every four years, and never improve, so this alone doesn’t explain Romney’s strength.

4. Romney is very intelligent. Romney’s superior ability to absorb and analyze information is the key to why he has improved as a candidate from 2008 to now. For that matter, compare Romney today to the 1994 version, when Ted Kennedy beat Romney to a pulp with his record at Bain Capital. Jon Huntsman tried that tack last night, and Romney crushed him. Romney’s domestic policy staff, while not widely known in conservative circles, is sharp and aggressive.

3. Moderators have not asked Romney a challenging health-care question. The key to tripping someone up in a debate is asking him a question he is unprepared for. It’s not obvious that those who have moderated the GOP debates are up to the task. Romney is well-practiced at answering the “Romneycare is different from Obamacare” question, and delivers his rejoinder with confidence. A stronger line of criticism — one that almost no one has employed — would be to point out how Romneycare has worsened the free rider problem, increased the cost of health insurance, and worsened emergency room crowding. Romney’s successor has been forced to try to clean up the mess by raising taxes and instituting price controls on insurance premiums. Even these points, however, are hard to pursue in a thirty-second format.

2. The eight-candidate, sound-bite debate format plays to Romney’s advantage. Because the debates must give sufficient air time to each of eight candidates, it has been difficult to ask follow-up questions and pursue the weaknesses in Romney’s well-crafted statements. The format of the Palmetto Freedom Forum would have been ideal for discussing health care policy, but that debate focused more on constitutional and social issues. For Romney’s rivals, it was a missed opportunity.

1. Mitt Romney knows far more about health policy than any of the other candidates. Health care policy is not a classical conservative concern. Romney, having made health-care reform the top priority of his governorship, knows the ins and outs of the issue in a way that no one else in the field does. The prominent Republicans who could have given Romney a run for his money — Mitch Daniels, Paul Ryan, and Bobby Jindal — aren’t running. Newt Gingrich comes closest to Romney’s health policy heft, but Gingrich has his own checkered history on the issue, and has generally stayed away from attacking the other candidates.

It’s hard to see how this dynamic changes between now and January. And given Romney’s strengths in most other policy areas, it’s hard to see how the race changes if Romney’s rivals can’t make health care an issue.