PlanetUnreal (thanks Gamer's Hell) points the way to the cached.net forums in which Angel Munoz offers the reasoning for not selecting Unreal Tournament 2004 as the game of choice for the CPL world tour, revealing that Epic is at work on a new installment in the multiplayer shooter series (as opposed to, say, another Jazz Jackrabbit game). Here's the deal:

People wondering why we did not pick UT 2004. First realize that Epic is a huge supporter of CPL. The CEO Mark Rein is a friend of mine and Cliffy B is on our advisory board.
A wide range of aspects were taken into consideration, including publisher/developer support for the entire tour. Unfortunately, Epic is transitioning to a new publisher and it was not expected that their current publisher would help us in marketing the game/tour through the end 2005. Additionally, there is a new UT in the works, but it was not going to be available in the time frame we needed.

Mark and I are still in conversations about the 2006 World Tour.