Because the point is to create a set of companies that autonomously explore and colonize space under the US flag, having multiple companies participate in the public option ensures at least one can continue on their own, ensuring further than nasa can always get a ride to wherever.

In an ideal world nasa would operate more like it's Chinese counterpart, but given the division in the US and the timeline required for space flight, that's impossible. The first manned flight beyond mars could be defended half way through the journey in the US with no real recourse.