NASA's budget is absolutely tiny in the grand scheme of things, it's no wonder they haven't undertaken anything really monumental in a long time.
I think space is fascinating, but unlike many I'm not so enamored by the idea of sending people to Mars. Even under the best conditions, it's a less hospitable and far more isolated environment than the most climate-extreme, desolate "corners" of the Earth. What exactly is someone who goes there going to do? I suspect the novelty of being on another planet will wear off pretty quickly once they get there. If an emergency occurs, they are on their own, even if we were to send them needed supplies it would take months for them to get there.
I would be more interested in sending unmanned robotic probes to more places. It's much cheaper and less risky, and we can send them to far more places, learning much more than we can by putting a human on Mars. Exploration can be mechanized and automated, the only reason to send a human there is because we can.