Particularly given that the mandate was already unpopular back when it was a mere freedom-crushing experiment in socialism. And particularly when Republicans had a bunch of other taxes in 'Obamacare' that they could just as easily attack.
On a couple levels, the weird rift between Romney and the rest of the party makes sense. Congressional leaders have different imperatives. They want to trip up Democrats, they want to win majorities, and they're under enormous pressure to build a legislative record that shows bipartisan opposition to key pieces of 'Obamacare.' From that arguably myopic perspective running with the tax attack makes some sense.
But my hunch is that the GOP prepared carefully for every Supreme Court contingency ... except the one where the law gets upheld as an exercise of the taxing power. Here's the RNC and the Romney camp taking a victory lap shortly after the decision came down. If they'd really prepared so well they wouldn't be dancing around the cable nets with two left feet.
Romney's health care history was always going to force the GOP into seriously contrived positions. But I'll admit I didn't imagine Rick Santorum's primary argument -- that Romney was "uniquely unqualified" to take on Obama and the ACA -- would bear out so exquisitely.