That's because NASA is a cumbersome, visionless, incompetent bureaucracy which would have a difficult time organizing a manned mission to break out of a wet paper bag.. Look for either the Euros or the Chinese to be the first to Mars, probably in a joint mission - with an honorary Russian thrown in for PR purposes..
NASA is simply not capable of building the infrastructure necessary for meaningful space exploration, or even encourage its development. Heck, you think DOD is bad with $2000 dollar toilet seats? With NASA every detail is a multi-million dollar publicity stunt. The fictional legend of the NASA pen and Russian pencil hits on a greater truth.