Difference is, I wouldn’t call Huckabee voters conservatives. Idealogical, single issue, social conservatives maybe, but not simply conservatives. While this doesn’t seem to address your whole post, it actually does - and it’s not necessarily a bad thing, it just simply is.
When candidates try to build a coalition, this is one group they need. When people talk about the Reagan coalition, this is one group they're talking about. When they say that you only have to win over conservative voters, they're wrong if they think you can leave this group outside the tent and still win.
There just aren't enough hardline freemarketeers and aggressive budgetcutters to win elections. Call them Human Events conservatives after the weekly that (very tellingly) ceased publication this week or Paul Ryan conservatives. There just aren't enough to get to 50%.
Even Reagan needed some unconservative Northern Democrats to win a mandate. And those in Romney's 53% (assuming his numbers are correct) who vote Democrat have to be made up for somehow with downmarket voters who are less enthusiastic about markets.
I suspect a lot of actual Huckabee voters did go to the polls for Romney, but the next group down the list, say, blue-collar White voters in Ohio, didn't give him the support he could have had if he hadn't made the comment.