Because you want to have competition to drive down the price. You want innovation. You have the ability to get people to agree that it's worth having a public plan. You could get private insurers to cover this population, but you couldn't without giving the population leverage in the marketplace.
Learn as many mistakes and what not to do while your business or product is small. Don't be in such a hurry to grow your brand. Make sure that you and the market can sustain any bumps that may occur down the road.

