There is still no word on why he didn't appear at the event but we have confirmed that he was in the bay area (San Francisco) earlier in the day. Also, Javier Lorenzo sent this one in: Hey Ryan. it's weird that John Cena didn't show up in Oakland because he was in the bay area during the day. I had the chance to take a picture with him yesterday at the San Francisco airport.
It could be a number of things - travel issues, elbow injury, family emergency, sickness, etc. These things happen and we're guessing he notified WWE management beforehand and it was cleared.
More as we get it.