Because it's not UT3 so much as Gears of Unreal Tournament Wars. They might as well have just plastered Marcus Fenix's oversized face everywhere.
The graphics were a huge step back from UT2k4. Part of the reason UT2k4 was so wildly successful is because the graphics were simplistic, even for their time. Colors were bright and, uh, colorful. Textures weren't overly complex and fuck bump-mapping. It was clear they wanted the focus to be on the action, not on the scenery. UT3 turned that on its head with overly complicated textures and muted colors.
The gameplay was either a step in the wrong direction or the right direction, depending on if you preferred UT99 or UT2k4 controls. Most every aspect of the gameplay went back to UT99, which as a veteran of UT2k4 turned me off.
UT3 also removed a lot of the most popular gamemodes. No more Bombing Run, I don't remember Assault being present, and Warfare just felt like Onslaught with a bunch of useless crap tacked on. They did add some interesting deathmatch modes - I loved Titan - but ultimately the best way to play UT2k4 was with teammates, not solo, and most of those deathmatch modes were basically for solo play. I'd play plain old TDM over any of them.
There's a reason the only PC shooter played competitively in any real aspect made in recent times is TF2, and even then it takes a backseat to all of the others. Modern shooters just don't have what it takes, and anyone who tries to babble about COD or Halo is gonna get lynched.