And the reason is this:
Human space travel is just prohibitively expensive!
It costs millions of dollars merely to toss a human in a tin can into low Earth orbit. Okay, maybe Branson Airlines can get the price down to a few hundred thousand bucks for a sub-orbital flight. But that's not a mission to a distant star. It's not even a trip to the freaking moon, our nearest planetary neighbor.
We're not going back to the Moon, and we're not going to Mars, and we're not going to Jupiter and Saturn and the little mini-worlds those gas giants host.
We ain't goin'!
Von Braun had the plans to go to Mars in 1956!
Our former idiot President G.W. Moron Bush announced that we were going back to the Moon and that we were going to be preparing to visit Mars. But of course he didn't give NASA the money to do these things. No, he was too busy packing our nation's wealth into the pockets of his daddy's butt-buddies. Trillions to commit mass murder in Iraq, nada for NASA.
So over the years NASA did its best to develop a half-assed system for getting us back into delivering fairly large payloads into orbit and to the Moon. It was called the Ares/Constellation system. Basically, it was a slightly larger version of the old Apollo module/Service module/Lunar module concept. Visually, it sucked because it had a tired "been there, done that" look to it. But if it worked once it could certainly work again.
However, nothing like that works if there's no money to buy it.
Enter the new political administration. At least they have been honest enough to admit that there's no money for NASA to return to the Moon. Our once-mighty space administration is now going to have to scrap the Ares project. The USA is hoping some private firm can get people into low Earth orbit. Or we can continue to hitch rides to the Space Station on Russia's tried and true Soyuz capsules. We've pissed trillions down a rat hole that leads directly into the bank accounts of a handful of powerful USA families. Want to know where the money went that was supposed to give us trips to the Moon and Mars; high speed rail travel from Seattle to Miami; universal health care; new National Parks; green energy? Look in the bank accounts of America's richest families.
They took it all.