I said this in another thread, but I feel it applies here since it pertains to the reaction:
I thought the reason Alberto won it when he did was twofold:
1) Recently turned face, and he needed a big moment on TV where more fans could see for free
2) The win came during a stretch where WWE is stopping in Florida, Texas, etc... essentially Southern cities with more of a Latino fanbase than places like, say, Minnesota or Cleveland. They may have been hopeful that Del Rio could get a warm face response in front of these fans (which he did), and hope that this kind of treatment follows him as WWE tours further North, away from the friendly audience.
Just my theory though.