In the quickest summary, SpaceX is structuring their business in such a way as to grow fast, get profitable, and remove the need for external funding. They are using COTS (government funding) in much the same way as a silicon valley company would use a VC to provide capital needed for rapid growth. They've been using that money coming in the door to create a non-government market and to make their technology very cost effective in order to grow the space market. Orbital has structured their COTS approach in such a way that they will be able to meet objectives and stay profitable, but it is not an evolutionary path towards a general space architecture.