NASA would gladly go for a Mars plan, workable or not, but if the President isn't willing to push for it, and Congress won't fund anything but pet projects and the bare minimum(or less) of what NASA needs to do it's regular work, there's little NASA can do.
NASA can't even convince Congress to fully fund the Commercial Crew effort, and that's far less expensive than any human Mars mission, and directly applicable to research, development, and operations experience that should precede a Mars mission.. Getting near enough budget for that has been like pulling teeth- I think the lack of interest you see in NASA has more to do with that agency being more concerned with just getting the funds it needs to complete the projects it has, rather than going hat in hand to the Congress with a considerably more expensive project than Apollo, one that every Congress since the dawn of the space age has been unwilling to pay for.