1. As Jonathan Bernstein has repeatedly argued, primary campaigns are about capturing the party elite, not capturing the popular vote. In foreign policy, the neocons are still in charge. The fact that neoconservatives have promoted policies not terribly popular within the base of the Republican Party, not to mention the country as a whole, doesn’t change the fact that they continue to dominate the elite levels of the GOP foreign policy apparatus. Romney’s hold on the lead is too fragile to risk alienating this elite by taking a different position on Iraq (even if he wanted to; I honestly doubt that he gives a damn). If there were a capable challenger on Romney’s right, things might be interesting; in this kind of race, trying to grab some of the 43% of the GOP that wants to get out might make sense. As we know, however, this is not the case.
2. No power comes with no responsibility. I have considerable doubt that any of the current GOP crop would have pushed very hard to keep troops in Iraq, given how unpopular the position is, and given how little strategic sense it makes. But then the lack of GOP influence over foreign policy means that critiques are effectively free; Romney can (within the bounds of the primary campaign) attack Obama all he wants on Iraq, without ever having to worry about dealing with the issue himself.
3. I very much doubt that Romney will challenge Obama on this during the general election. Apart from the thin crust of GOP foreign policy elite, my sense is that even the “pro” stay in Iraq faction doesn’t consider the issue all that salient, while the “anti” faction in the Democratic Party (and probably among independents) seems to consider the issue pretty important. It really does seem that foreign policy is going to be a political strength for Obama (setting aside actual evaluation of his foreign policy), although I doubt that it will be all that relevant to the outcome. It’s also pretty clear to me that the decision to withdraw active duty US forces was correct both politically and on the merits.