I agree with Xtreme, there's no reason whatsoever why MVP couldn't win MITB while holding the US Title. In fact the best thing about this years edition is that so many of the entrants could win it. Ok, Mark Henry's got no chance and the day Kane gets a push is the day hell freezes over, but MVP, Punk and Christian are all in with massive shouts (despite Punk's failure to hold the title successfully) and even Shelton (maybe he'll finally get the new push that a lot of people think he deserves) and Kofi (grudge match vs Edge) have outside chances.
As for why Christian vs Edge hasn't happened, most fans don't give a shit. Christian was an upper midcarder who occasionally main evented several years ago. He then left for what is, for all intensive purposes, a developmental territory to the WWE and the majority of fans. Ok, us lot on here know he did a good job in TNA and showed he can carry a world title, but do you think a 12 year old wearing John Cena wrist bands knows that? No, of course not. And he'd have no reason to. I certainly wasn't watching what is basically an indie promotion when I was 12. Christian is - at best - a guy they vaguely remember and - at worst - a completely unknown quantity. And make no mistake about it, these are the fans the WWE is listening to.
I suspect the WWE let him go to see if he could become the star he claimed he could, and frankly I think he managed it, although that's obviously up for debate. However all of his achievments in Florida count for nothing, because the majority of WWE fans don't know about them. Now that his previous employers have seen him as a world champ they can get to work pushing him towards that spot in their company. Maybe it'll work, maybe it wont, but I can gurantee that if he'd of just strolled in and won the title most people would not have cared.