Another factor may be that fewer people are going to multiple shows.
From 2005 to 2009, I would attend two or three Dream Theater shows every year. This made sense because the tickets were not very expensive ($45 to $55), and they changed the setlist every night.
In 2011 and 2012, tickets are considerably more expensive ($70 to $80), and the setlist does not change every night. So there's no reason for me to go to more than 1 show.