(You can blame Kurt Angle for this one.)
From a post on shane-carwin.com titled "UFC 108 Not Happening For Me":
Heavy.com confirms that Lesnar has been struck down by what teenagers refer to as "the kissing disease," and not the flu as originally reported. There’s currently no timeframe on when Brock will be healthy enough to return to competition.
Unfortunately, this means UFC 108 is currently being headlined by Rashad Evans vs. Thiago Silva. But there is hope: To shore up the January 2nd card, a heavyweight clash between Cain Velasquez and Antonio Rodrigo Nogueira may be in the works — which would surely determine who’s next in line for a title shot after Shane finally gets his turn.