Jonathan Chait on why Obamacare has been as successful as it has been, which is not to say it's been as successful as it could be. Here's a section that strikes me as getting to the meat of the problem. After noting that the two parties could work together to make it a better, more efficient system, he says…
Republicans, and Democrats can't compromise on health care for the same reason they can't compromise on taxes: They have diametric goals. On taxes, Republicans want to shift the burden from the rich to the poor, while Democrats want the opposite. A similar dynamic exists in health care. Republicans want to restore the ability of healthy and wealthy people to buy cheap plans that don't cross-subsidize the sick and the poor — the very features of the insurance system that Obamacare was designed to stop. Since Republican ideas for improving the health-care system all involve shifting costs from the rich and healthy to the poor and sick, there's just no way to blend them together with the goals Democrats have in mind.
And of course, you have the added problem that some people just hate the idea of something Obama did succeeding after they fearlessly predicted/pledged nothing would. They yearn to stop it, no matter what that does to human lives. But even as things are, a lot more people have health insurance now than did before and the costs are less than they would have been without it…or the still-non-existent Republican alternative.