Because if it WERE, the public would eventually get their hands on it, which means millions of people would have the ability to change history to their will. Now, eventually, some idiot/crazy person/terrorist is going to go back in time to where the first humans were, and kill them, or something along those lines. That would cause a major paradox, seeing as the killer's ancestry line would never exist. So as long as the universe as we know it isn't destroyed, we can be certain that time travel will never be possible, at least for humans.
This isn't the only example to prove my point, there are an infinite amount of possible paradoxes, thanks to the butterfly effect.
Your thoughts on this matter?