I don't think as many people were opposed to it as made to appear on Big Media infotainment outlets.
Polls showed differing numbers, depending on how the questions were asked (even more deviant than normal) and the big "NO" polls were asking in a more or less roundabout way about a government takeover of healthcare, which Obamacare is most certainly not, so the Democrats went for it.
The people that are screaming about government taking over healthcare are/were already going to vote against Democrats out of ideology, it's why they so readily believe the lie that anything this President or this Congress has done so far is "socialist" or even more hilarious "communist".
Personally, I think Obamacare is a joke, but for pretty much the exact opposite reasons that people were railing against it. It's uber-capitalism (well, uber-modern-capitalism anyway, we all know it's different than Adam Smith's vision) in an area where I believe a more social touch is needed.