Two reasons, actually: Democrats did the work, and Republicans didn’t.
An article about how Obamacare was put together, and how it survived the recent Republican president. (The article is adapted from a book.) 3,500 words:
Obamacare’s very survival seemed so improbable just a few years ago, when Donald Trump won the presidency. Wiping the law off the books had become the Republicans’ defining cause, and Trump had pledged to make repeal his first priority. As the reality of his victory set in, almost everybody outside the Obama White House thought the effort would succeed, and almost everybody inside did too.
One very curious exception was Jeanne Lambrew, the daughter of a doctor and a nurse from Maine who was serving as the deputy assistant to the president for health policy.
I found especially amazing the amount of work needed, by thousands of people over several decades, to get a major piece of legislation passed:
It demands unglamorous, grinding work to figure out the precise contours of rules, spending, and revenue necessary to accomplish your goal. It requires methodical building of alliances, endless negotiations among hostile factions, and making painful compromises on cherished ideals. Most of all, it requires seriousness of purpose—a deep belief that you are working toward some kind of better world—in order to sustain those efforts when the task seems hopeless.
Democrats had that sense of mission and went through all of those exercises because they’d spent nearly a century crusading for universal coverage.
Also interesting was that the previous President apparently could not understand that pissing off a powerful Senator might not be a good idea. That one Republican Senator, John McCain, prevented the repeal of Obamacare.