Because it hasn't undergone the proper level of review in order to be published in one. As it stands, there is no reason to take it seriously until it has been. There have been several posts of such papers here before, and I would highly advise the level of skepticism expressed by a fellow HNer with submissions like this c.f. : http://news.ycombinator.com/item?id=893877
Granted, but this author does not set off any crackpot flags: he's had plenty of prior publications in areas relevant to the proof and works in an industry research lab. Another poster points this out: http://news.ycombinator.com/item?id=1585999
Also, the paper was written using LaTeX. I have a friend who spent a semester helping to edit a small mathematics journal, and, with virtually 100% accuracy, you could tell the crackpot papers from the serious ones based simply on whether they used LaTeX or not.
Warranted. I just despise hype and sensationalism, particularly those concerning scientific breakthroughs. The title of the submission echoed similar hollow, dramatic claims and triggered my skeptical response.