Because it was demonized as 'Socialism". And private health insurance companies lobby like crazy to make sure it continues to be seen that way.
Back in the 1930's employers began offering health insurance as an employment benefit and it took hold and never went away. If you work for a large company that provides good insurance, it doesn't seem like a huge issue because "I got mine". Until you get laid off that is, or the company decides to reduce benefits. So you have a system where if you're employed in a decent job you're covered but if you are ever in between jobs, you pray you don't get sick. Another objection was people being told they won't have a "choice" if we had Universal care. But no one really has a choice as it is. Your employer is contracted with 1 company usually. Take it or leave it. The only choice i've ever had is between a standard or premium plan. Your yearly deductibles and co-pays are a little lower with the premium but of course you pay more per month.
I have pretty good insurance through my employer and had a minor (as in non-life-threatening) operation 4 years ago. I still ended up with a bill for nearly $3,000. Navigating the co-pays and yearly deductibles is a pain. I can't imagine trying to deal with it if I were debilitatingly sick.
The strengths of our system are we have some very good hospitals and medical schools. Great if you can afford it. I remember during the debates over universal healthcare the republicans always loved to point out that rich people from other countries like to come to America for healthcare. Yippee for us I suppose? Rich people also like to build homes in Beverly Hills, so what?
Obama Care as I understand it was a crappy compromise. It provided a marketplace for people to buy private insurance at different price levels(thus different levels of crappiness) without being denied for pre-existing conditions. Making it available doesn't really help a low wage worker who's barely scraping by though. Also at tax time there are penalties if you don't have insurance.
"here's how we fix the problem of poor people not having insurance. We make it illegal to not buy insurance" Brilliant.
As to why some hated Obama Care so much? Because it was Obama. And we were assured he was the worst President in the history of forever and probably wasn't even really an American. Before that, Bill Clinton was the worst President in the history of forever. Now they assure us it's Biden. In my grandparents day it was Franklin Roosevelt who was the worst man in the history of forever.
We do have Universal healthcare of a sort. For the elderly. Everyone qualifies at age 65 for Medicare.