I'd say it just makes it impracticable at this time. I don't think technology in general has reached the point where we could do it with a reasonable margin of safety let alone cost. The other thing is that for what we're doing right now, unmanned probes can get the job done at less cost with no risk to us. It's not that we can't strive for manned missions but there's not necessarily any reason for it to be a high priority.
"Don't confuse facts with reality."
-Robert D. Ballard