Actually, I think we ought to let Scaled Composites do it (not NASA). Burt Rutan will get it done at a 10th the price.
No one's stopping him.
Have him bring some ideas and investment to the table and NASA will listen.
The guy built a fancy plane that barely kissed what is technically defined as "space". He's never achieved orbit, which is an order of magnitude more difficult, and you're ready to say he can make it to Mars? I'm not arguing that NASA is efficient or anything, but they actually have a proven track record beyond Rutan's solitary stunt.
That being said, I do want private enterprise heavily involved in our space exploration. I'm just have not seen enough evidence to tell me that Rutan can even achieve orbit, let alone lead us to Mars.