Alternate headline, NASA spaceflight review states the obvious about the lack of ability for the agency to get humans to Mars.
There are two major reasons this is so.
1: Lack of vision. The Apollo program was successful because we had motivation for going, a time table for when we'd get there, and a United nation behind the effort. At the moment, at most we have agreement that the space program is important. Less agreement on what it should do, and how much we should fund it. With various congress people putting pork before everything else. IE: SLS is too expensive to be very useful, and its a big rocket without a clear destination while it consumes resources that could of been better used elsewhere.
A lack of vision is frankly the American people's fault. Too few of us value science or exploration, and we are too quick to be critical of big numbers without looking at the big picture. Example: 500 billion to go to Mars?! Outrageous! Even though that is less than what we spend on defense in one year. Not to mention that 500 billion would be over 20-30 years. Which would be less than American consumers spend over 20-30 years on just alcohol and drugs alone. Less than we spend on our pets over 20-30 years.
To be fair though, NASA has been without a clear vision since the end of the Apollo program. Indeed, one of the the reasons the shuttle and ISS became such a boondoggle like SLS is because of that lack of vision. We need to be honest with ourselves on why we want to send humans into space, and realize that its perfectly OK to send humans into space just to test the limits of what we are capable of as a species. Unfortunately, we've been too focused on the bottom line to dream about the future anymore.
Which is why Space X is so amazing, they are doing something we haven't done since the 60's. They want to go to Mars, because why the hell not? That honesty has given them a very clear path to getting to Mars too. Which is why NASA should just back their vision if Space X wins the commercial crew program contract as I believe they will.
2: Money. NASA's budget has been going down for the most part since the 70's, they simply do not have the resources to do ambitious missions like Apollo, let alone the next Curiosity or Cassini. That is a fact.
After 2020, we wont have any outer solar system unmanned probes exploring the outer solar system for the first time since the 1970's. Worse, the future of exploration is the outer solar system. Yet NASA had to kill its next generation Radioisotope Generator last year. Meaning we killed a generator that would of been more efficient/cheaper in the long-run, and allow more outer solar system missions. All to save a little money last year.
The money simply isn't there for a significant manned mission, let alone a significant unmanned mission. Which is just depressing. We used to be a nation of explorers, that embraced the unknown. Now we're a bunch of anti-science idiots who only care about ourselves.