Because debuting him at the PPV would have been stupid. They already had JJ's return and a Sting apperance, they didn't need to debut Foley too. I'm more surpised they didn't do it.
Why? The two have had little to no interaction over the past few months wheras Joe and Sting have. Sure, AJ/Joe would be the better match, but that doesn't really mean it should be the main event. Sting/Joe makes the most sense at this point, and for the first time we will see the champ retain at BFG.