It's Chekhov's Gun, really. If you bring a gun into the story in act one, it must fire in a subsequent act.
They brought Rock in for WM27, but they couldn't just let things slide. Not only would people probably criticise the loss of having something big between Cena and Rock, it also wouldn't have made sense for the two to NOT have a programme of sorts running into it. The Rock is way too big a character to just have host a WM without any controversy flowing from it, and Cena is this era's Rock. The two must clash.
It's a damned if you do, damned if you don't scenario. WWE didn't really have a choice. They could have had the programme continue into the next PPV, but let's face it, we haven't had a big enough PPV name for such a clash to happen in. The rumours are that it was originally slated for Summerslam, but even that would be too far away. How would WWE keep the interest big enough for Summerslam? Have Rock interfere in Cena's PPV matches and keep costing him the title? That'd get pretty boring pretty quick.
With Wrestlemania, however, the resparking of the feud will be a lot easier. It's in Rock's city, and let's face it, Rock's going into the HoF. There's a whole lot of programmes that can work from that.
I'm not sure when WM tickets go on sale, but I'd think you could pre-order them, right? Correct me if I'm wrong, however. This feud is seen as guaranteeing a well-sold WM28, in WWE's eyes. The sad thing is just how blatantly apparent the fallout of this decision was. But, like I said, WWE didn't really have much choice in the matter. I'd have much rather had their match down the card, but in WWE "main event" can mean anything, now (what with Del Rio's RR win securing him a main event spot opening the card <_<).
It would have been a lot better of a decision than merely dropping the storyline out of nowhere like WWE has been doing lately. After all, how "stale" has this storyline really got if we still discuss it to this day? People still talk about it, and that's the main thing. I, personally, don't wish to really see this match, but it is something of major importance so I'd think we should wait to see how it's booked closer to the time. WWE might pull out all the stops, or they may fail us once again. It's just a matter of waiting it out.
At least something has been built from this, and we're guaranteed a historic match. Out of the monumental cock-up that was everything, WWE has, imo, made the best decision going forward. That being said, Rock should have just simply Guest Hosted WM28, and this whole ordeal would never have happened.
Also
I'll tell you why.
You just started a thread about Cena vs. The Rock. Well done, you just answered your own bloody question.
The reason WWE made Cena vs. The Rock a year in advance was so that the event could be hyped up by The Rock making appearences throughout the year, and most people I've seen interview him have asked him about Cena and The Rock. Therefore the word is getting out, It'll continue to be passed on over the course of the year so that when WrestleMania time rolls around which I know for a fact, a lot of people watch, even though they don't watch throughout the year, WWE will end up getting more pay-pre-view buys and the match will have a certain degree of hype built around it.
There are currently two other threads with discussion on The Rock/John Cena rivalry going-on, between Cena and The Rock talking back and forth on Twitter that is an avenue to keep people entertained. Although they will only have interaction for the final quarter of the Road to Wrestlemania I assume, Rock will still make spariodic appearences which will possibly, in what WWE is hoping, draw viewers in.
The idea is working. The Rock vs. Cena is several months away but people are talking about it, you are talking about it hence why you set-up this thread which is proof that it's working an asides all that they did it because the feud needs to end, and putting a match like that at say, Extreme Rules which was the pay-pre-view after WrestleMania would have been ******ed and The Rock hasn't wrestled in over seven years, that alone will make anticipation, hype and excitement build and once again it will draw viewers predominantly from the Attitude Era back into wrestling which will highten WWE's viewers and PPV buys.
Booking Rock vs. John Cena a year in advance might have the odd disadvantage but it has much, much more advantages. Therefore in-terms of business it was a smart move.
THIS