NASA is limited by the idiots in Congress with their Senate Launch System. Forced upon them by the likes of Shelby and other politicians whose districts have jobs that relied on the Space Shuttle program that is now retired. The SLS and Orion capsule only exist to keep those people employed by recycling as much components from the Space Shuttle as much as possible. It's a rocket that will only launch once a year if that, has no mission, no payloads, etc.
If NASA was able to design a rocket via open competition, I bet we would see an entirely different rocket, or hell NASA may decide to use SpaceX's Mars transporter rocket instead.
Congress also refuses to fund the components needed for a Mars mission like a habitat. NASA's hands are tied.
