WWE's reasoning for Christian not being apart of WWE Main Event tonight was that he had in fact suffered a concussion on Raw the night before. 

We think we can identify it to a kick from Alberto Del Rio that Christian took in the Fatal 4 Way #1 contenders match yesterday. Christian looks legitimately dizzy and confused after it. There's no real reason WWE would create a small storyline unless creative plans changed at the last minute so it does look like a legitimate concussion.