It's an election year, no one wants to vote against it. The Democrats want to take control of Congress, and Obama can afford to take a hit. So they let it go to him (and contrary to reports, it wasn't a unanimous vote. It was unanimous consent, which is different), then when he vetoes it and it come back, they narrowly repeal it by denying a 2/3 majority. Hopefully late on a Friday when the news cycle will flush it out.