I have the scoop.
The ITW conference committee orginally expected 40 - 60 writers to participate in panels and book signings. (wow, that's a low figure for a conference.) We were all given a three-title limit for the book room. They would supply the books as long as the titles were still in print. Now they have more than 150 authors on panels. In order to cut down on the amount of books that need to be transported, the committee decided to allow 2006 titles only. But this is June.... :D