According to this NY Times article, Warner Bros., Disney and several other movie studios are pulling out of SDCC. Apparently they don't feel they are getting their monies worth.
https://www.nytimes.com/2011/06/13/...l=1&adxnnlx=1307962877-OecLSVZdaDdSYXJbXoJszQ
maybe it's because they don't have anything near completion to talk about or show?