That’s because republicans and Astro turf movements were hell bent on protecting health insurance companies.
Actually Healthcare providers, Pharma, and Health Insurance companies spent much more time & money lobbying Democrats than Republicans for ACA. These companies wanted to be sure that the U.S. government did not go with a "public plan" and these lobbyists basically wrote all of ACA as a giveaway to their industries. I will note that it backfired on Insurance companies in 2014 when the government took away "risk corridors" which guaranteed them a profit on all ACA plans. The reality is that either the U.S. needs a public plan (similar to Germany, U.K. Canada, etc.) or they need to completely roll-back ACA & go what existed prior to ACA. I actually support driving for a public plan -- even with all of its problems and waiting times. As long as it is modeled on Germany's where you can opt for a private provider (via your corporation) and out of the public plan.
Totally agree with you here. This is one of the places where the government can extract efficiencies through scale.