Believe it or not, I'm told WWE officials have actually soured on working with NBA legend Shaquille O'Neal over negotiations for a potential Wrestlemania XXVIII match between him and Big Show. This is what was responsible for WWE announcement last week that there were no Wrestlemania plans for him. From what I understand, Shaq wants to go over, which may at one point not have been a deal breaker, but is now what is responsible for holding things up.
There are many in the company that were against having Show put over Floyd Mayweather a few years back and there have been a "very vocal few" who feel Big Show should not go under to another outsider at Wrestlemania. Keep in mind he also put over Akebono at Wrestlemania 21 in a sumo match. In the case of the Mayweather match, the company was desperate for a mainstream draw for the show and it was considered to be worth sacrificing Big Show.
The situation is a bit different now though. Not only is WWE not desperate for a big outside attraction since they have The Rock, but the depth situation being what it is in the company these days, Big Show has been considered a higher priority the last couple of years and the company has been more careful about protecting him.
Time is starting to run out on finalizing a deal with Shaq and if a deal can be struck in the next week or so, the match could still happen. At this point though, WWE is ready to move on and feels between The Rock vs. John Cena and Triple H vs. The Undertaker Hell in a Cell is a pretty strong one-two punch.