#2 Their history together
This is the most obvious reason of all, isn't it? Simply put, the two have a history together and WWE may have brought Edge back and put him right away with the most familiar man on the roster. After all, the roster has gone through a radical change since he left and has been filled with Superstars who came up only after he retired.
In that sense, there's a sense of familiarity. Not that Edge doesn't want to compete against the young guns of today, but the reaction that the Edge-Orton interaction had on both nights were great.
At the Royal Rumble, it was surprisingly his interaction with Orton throughout that got the loudest reaction. They were certainly listening carefully to the reaction as well and you can believe that they did the same when Orton offered a Rated RKO reunion on RAW.
It was a clever ploy to get heat and one that worked well. While many of us may complain that we would rather see Edge with a new opponent, a large part of the audience will be happy to see this match.