The reason is pretty simple. The health insurance reform that Romney helped pass and signed while Governor of Massachusetts is very similar to the bill passed by the Democrats. The cost controls aren't the same. And there are some things the federal government can do that a state government can't. But on the big picture, it's pretty much the same thing.
If the Republicans want to make Obama's signature piece of legislation a centerpiece of their 2012 campaign (and it's hard to imagine they won't since what else will they run on?), they can't very well run a candidate who supported and passed close to an identical bill. It's a no-brainer.
I too don't like broad and blanket statements about what has to happen two or three years down the road. But can anyone explain to me why this doesn't fatally hobble any GOP presidential campaign in 2012? And because of that make Romney close to impossible to nominate?