There's a very simple reason that Obamacare hit 8 million sign-ups: Being uninsured is horrible.
But the political conversation over Obamacare was driven almost entirely by people who had, and knew they would be able to keep, their health insurance. It was filled with a lot of assumptions, theories, and speculations about what people who didn't have good insurance, or any insurance, would do. And after Obamacare's disastrous launch, the theory took hold that these people wouldn't find this untested program worth the trouble. It was the permanently insured speculating about the uninsured and the barely insured – and, unsurprisingly, they got it wrong.