Just a personal opinion as I'm not aware of the overall trends but what stops me desiring to play Ut2004 online now is the "Pro" attitude mentioned before. That and the fact that there is no variation in gameplay. I get a bit sick of Rankin, Deck17 and Campgrounds because the gameplay does get old quickly.
There is gameplay flow on these maps yes, but with trick dodging etc so much of the brain isn't utilised and I often feel like I'm on some kind of conveyor belt that will lead me to the action no matter what I do. This is a good thing when it makes action accessible but without enough player interaction with the environment it can become boring because there is a lack of strategy (eg. someone disappears through a door that should provide a defensive position but doesn't because there is a multitude of ways to easily cut them off).
I think the reason the Unreal/UT series took off was because people could be creative and put their ideas about what constituted fun gameplay online. Perhaps most of the people of that inclination have moved on, as gaming online is boring as hell with the exeption of a few small interesting servers.
The attitude that there is a "correct" gameplay flow persists on all of the map sites I've noticed. Rather than building maps for a community the focus becomes building maps that conform to a series of statistics to get a good rating. I realise "fun" is subjective but I think there's more potential to the game than the high-tech tournament style flow of the retail maps.
New maps are flamed on many of the review sites if they do not conform to a particular gameplay type. I think this causes UT2k4 to stagnate at the least. If someone comes out with a gimmicky map that lacks gameplay, fine, but there is usually some way to refine the gimmick even so that it caters to the self-proffessed "hardcore" players. I've recieved more positive feedback on my last map than expected so please don't mistake my words as being a gripe, but I think a more adventurous attitude would be the only way to replenish the UT communities. Having played newer people at the game I find they're put off because of the difference in skill level. Unlike some other FPS games, camping etc are discouraged by the very environment, so there is little opportunity for strategy to combat skill difference. Until environments exist where new players can still get one over by the more experienced through tactics and deception the game will always seem to a newbie like they're stuck in a place they can't escape with certain death.