Of all the evergreen keywords, flying is one of the ones with the longest heritage, one of the most prevalent... and also one of the most powerful. It stands to reason that reach is also a necessary keyword so that the colors with not much flying have a defense against those that do. Green already has plenty of reach; but, say, black and red do not. Mark Rosewater took on the topic of secondary colors for reach on his Tumblr blog recently. When planeswalkeratog asked, "Is reach an exclusively green mechanic?" Rosewater answered:
Currently, but I think we could branch out into a second color, white and red being the obvious candidates.
Now, red got reach for its anti-flying theme in Planar Chaos, and Hundred-Handed One can gain reach for flavor reasons; but of all colors, red easily needs reach the most. White already has flying, after all.
Audience reaction to this was mostly on the side of red getting reach in the future; drkobolds wrote "My initial reaction was that Red shouldn’t get it because it is a pure defense ability, but it is good against red’s two enemies. So, maybe?" while chainsofmephistopheles said "I feel like blacks recent change towards higher toughness would fit well with reach. Black Oak of Odunos from BNG is a great example of the type of card black now gets for reach. On W/R reach would feel less defensive, which is odd for a purely defensive mechanic."