I believe it is because this is the night of the Pop Warner Super Bowl Celebration Party at MGM Studios and the park may be closing a little early.
It is an affair with a separate admission fee.  It is included in the hotel/park hopper package that all Pop Warner participants must purchase in order to compete.  Therefore, it is really swampped with Football players, Cheerleaders, family, etc. letting off steam in one last hurrah before heading home.  
Saying this, it should be a great night at the other parks as far as low attendance!