Unreal engine at that time was capable of rendering huge areas compared to Quake 3 engine but that resulted in blocky graphics. When they improved it and released UT2003, they went overboard with meshes creating detailed organic shapes everywhere and that way changed the look (also made it cartoonish in a way). They also added Mr Crow and other "funny" characters you could choose from, while in original UT all combatants were sporting futuristic gladiator look. Both those things were fixed in UT2004 to a degree. But when UT3 came out, Mr Bleszinski decided that UT should share similar look and feel with Gears of War. That decision resulted in complete organic overhaul of art style and game lost it's industrial touch. On top of that it was bloomy and blurry like hell on default.
Now when Cliff Bleszinski is out of the picture (left Epic awhile ago) designers can go back to original style of the game, which was one of the reasons why series are successful and well known.