#3 An unpredictable finish to a predictable PPV
WWE knew that they had to plan something big for Randy Orton vs Edge. Not only did they add pressure by giving it "The Greatest Wrestling Mach Ever" tag, but they had to surprise fans with the outcome.
Many believed that Edge winning was all but certain. After all, there wasn't much incentive to have him return and lose only his second singles match in 9 years. However, the storytelling alone was a reason to do it.
As mentioned, many fans including us would have preferred to see Edge feud with a younger star, but we can't blame WWE for wanting to put two major stars together due to the clear lack of star power on the card.
The finish came off as an unexpected one and you could argue that Orton winning itself was a surprise. Given how predictable the overall Backlash PPV was, WWE needed something big to end the show. And this was how they decided to go about surprising fans.