The ACA was more or less the GOP's healthcare reform plan[0]. They fought so hard against it because they didn't want the Democrats to get credit for it. The ongoing animosity toward it from Republicans is ridiculous, and Democrats are even further from being able to pass any more healthcare reforms than they were when the ACA was passed. The all-too-brief excitement for Medicare For All is somewhat emblematic of that.
[0] To be fair, it did go further than previous GOP proposals. They did include individual/employer mandates and a marketplace, but not stuff like the Medicaid expansion and higher taxes on high earners to help pay for it.
[0] To be fair, it did go further than previous GOP proposals. They did include individual/employer mandates and a marketplace, but not stuff like the Medicaid expansion and higher taxes on high earners to help pay for it.