The David v. Goliath thing has been done to death. Heck, Bryan did that last year, beating Helmsley, Orton, and Batista in one night with a bum shoulder.
So has everything in wrestling. That's just one more reason why nothing means anything anymore.
It has been done before, and it'll be done again, simply because it works. The whole point of wrestling is to get the crowd emotionally involved in the show, make them come along to the events, buy the PPV's, buy the merch. (And in WWE's case, keep the stock price up...!) They've already got everyone invested in Bryan, he's more over than anyone else in the company, the fans want to see him do well, to see the underdog win. Yes, they did it last year at WM, but then he got injured and had to relinquish the title.
Letting him scrap with Lesnar at WM, and potentially win back the title he never lost is the perfect story, right along with the happy ending. Nobody goes to the cinema to see the hero fail and die at the end of the movie, you go to see them succeed, to overcome the odds.
That all said, Reigns did very well last night in the sit down interviews, I was impressed with him. Him and Brock could be a great match, but Vince/HHH's decision has made his job so much harder now, when it didn't need to be. They knew that the fans would react that way, why else would they have dragged Rock into it, so that shows that they deliberately went against the fans.
Maybe they've got something up their sleeve? Or maybe they're just a pair of stubborn SOB's who don't like to admit they got it wrong...