First of all, as Speaker Boehner so accurately noted, Romney and the Republicans want to use the struggling economy as a way to make this election all about a referendum on President Obama rather than a choice between he and Romney. In other words, they know Romney and his policies can't stand up to voter's scrutiny. So they want to avoid that at all costs.
They also know that there is a very deep chasm between what Romney did as Governor of Massachusetts and what he said in the primaries. Since they need both their base (who need to hear what he said in the primaries) and independents (who would likely be drawn to what he did as Governor), they can't afford to let that chasm show itself. Better to just let people project what it is they want to see.
In order to understand the second reason, we have to take off our political junkie hats and think like a low-information voter. A recent New York Magazine article about the Democratic SuperPAC Priorities USA laid that out very well. It might be hard for people like us to believe, but here's something they learned from a focus group.
What became clear was that voters had almost no sense of Obama’s opponent. While conducting a different focus group — this one with non-college-educated Milwaukee voters on the eve of Wisconsin’s April 3 primary — Burton and Sweeney were surprised to learn that even after Romney had spent months campaigning, many in the group could not recognize his face, much less characterize his positions.By now, most Americans have made up their minds about President Obama. But as late as last April, Romney was still pretty much a blank slate. That's just how he and the Republicans would like to keep things. Its our job to make sure they fail.
Overall I agree with Jonathan Chait that this is stage one of a two-stage strategy.
Once they’ve established that frame for voters to understand Romney, then they have set the stage for a closing attack that focuses on the policy contrast.First, we make it a choice between two candidates. Then, we compare their records/policies/vision.
I suspect the switch from stage one to stage two will happen in the fall at the Democratic convention. Then its "game on" to the debates and the home stretch.