The Federal plan operates like a big market. Each year, you can change between plans from a menu of hundreds of plans. You can also see other people's reviews of those plans. Over time, plans that can't compete drop out. Other plans get better and service gets better in order to compete.

Prior to the ACA, health insurance was heavily regulated. You could not buy across state lines. You could only buy (affordably) within the plans offered by your employer. Individuals oftentimes couldn't even buy plans (for any amount) that were offered to groups. With the ACA, this is even worse, but that's beside the point.

Creating a single market with the Federal plan would have removed tons of anticompetitive regulation and enabled anyone, regardless of who they worked for or self employed or whatever, to buy a plan from that single market.

Moving the tax incentive to people instead of corporations would have allowed people to pay for those plans with pre tax dollars.