Well, there could be many reasons fof playing Rankin-Cheops-Campgrounds-Under in circles. For example, I'm a big fan of DM-Goose, and it was even re-made in UT3. But the server only has a "tweaked" version, which people don't seem to like.
The night version of Pipes is the same as regular, its just easier to see the players there.
The Noxious Fumes was really the most popular back in 2007 on NDK clan's server.
Calandras is a small map, a fun alternative to Idoma.
And yes, I noticed the fact that most popular maps are still the same as 10 years ago, which is great, because I know them well!