Because if there is a public option funded by taxation (whether you use it or not) and a comparable or even marginally superior private option which you have to pay use fees for (on top of the taxes), the public option will win every time. Simply put, do you want to pay once or twice for the same service? It's not like one car taking the toll road instead of the public road reduces that driver's tax burden by any noticeable degree. In antitrust terms, the government is using its monopoly in one area, "protection" (mostly from itself, as with any protection racket), as leverage to create a monopoly in another area, transportation. The surprising part is that there are some areas where the public roads are just so bad that you can actually run a profitable toll road alongside them. Level the playing field by requiring the government to fund the construction and maintenance of its interstate highways exclusively through use fees and I expect you'll see a very different result.

It's the same with any other bundled service. Having decided to subscribe to a certain service package one tends to stick with the included services, even if on an individual basis there are better services available at lower cost elsewhere. It only becomes viable to switch to the better service provider if you can replace all the bundled services. Except in this case that isn't even an option since there is no opting out of (paying for) the tax-funded package.

Who knows, it might have even achieved a reasonable balance between the two, instead of forcing a car-centric environment everywhere like the public option? ;)