There's been a lot of speculation as to why Alberto Del Rio was made World Heavyweight Champion, but the real story behind the scenes is that Big Show could not come to terms with WWE on a new contract. Since Big Show's contract was running out, WWE Chairman Vince McMahon made the decision to crown Del Rio the new champion. At the time of the taping in Miami, that was actually considered potentially to be Big Show's last match in WWE. Later in the week, the company and Big Show came to terms, and he hasn't missed any shows.
We heard from one source on the writing team that the company was very angry the negotiations went all the way to the breaking point, but now that Del Rio has been crowned champion "the plans have changed, as Del Rio is the focus of the show going forward."