a) it doesn't matter when they made the content. What matters is when the game was released. You are comparing it to games of its time and from those other games you take the release date too and not a random date during its development cycle.
b) UT2004 looks a lot better than UC1 and I'm not even talking about the resolution. It's not as apparent in the maps UT2004 still has from UT2003 which are also in UC1, but even in those the textures are a lot sharper. There is even a difference between UT2003 and UT2004 although it's a lot smaller than the one between UC1 and UT2004. Again, the textures are slightly sharper and the maps that are new in UT2004 also pack more graphical punch than the UT2003 ones.
c) thus you can't just take UT2004 and say it is a 2001 game just because a twin product of its predecessor was in development in 2001.
You can compare the looks of UT2004 and Left4Dead and maybe argue that Left4Dead doesn't look
that much better (although it's hard to compare as they look nothing alike), but that doesn't mean Left4Dead looks like a 2001 game.
To get back to your original train of thought:
I also prefer UT3. I think it looks fantastic in every sense of the word and I enjoy how it plays. I also enjoy it as a co-op game and as bare-bones as the campaign may be, my brother and I are on our third way through it (Normal, Hard and now Insane) and we play Instant Action together and against each-other as well.
I like the idea behind Left4Dead's survivors but the game itself never really appealed to me enough to make me want to buy it. Most likely not a bad game. Just not something I am interested in at the moment. Its popularity is probably due to it being a zombie game in a time when zombie games and mods were extremely popular; it being a co-op game when good online co-op games were relatively rare (especially on the pc); it being very polished and it being released by a reputable company as Valve. As it's co-op the peer pressure to get it ("you must get this game so we can play together") was probably higher than average too and with its low system requirements and its non-full-price price the threshold for people who were on the fence was a lot lower as well.
All in all it was the right game in the right time.