Fatty.. I assume that you are referring to public servers. Obviously, the large # of Rankin-only maps means that these are demo servers. Because people can play on these without spending a penny, they tend to be populated most of the time. From a more serious standpoint, Rankin is actually a VERY good map for 2v2 TDM... Any more than that, and it gets stupid.
As far as public play, DM and ONS have always been the 2 most popular gametypes (in terms of player/server counts). TDM & CTF had always trailed behind these 2 by a fair amount. From a competitive standpoint, TDM, CTF and TAM are the stronger gametypes.
TAM has been the most recent popular gametype to play, as it manages to be both accessible and fun for newer players (no small feat in the 2k* world). ....And as Houdini said, it has a social aspects to it as there is down time from playing.
Regarding general trends, more realistic shooters (WWII, CS, etc...) have been dominating online shooter counts for a while now (the days of UT, Quake, half-life style games have declined). With 1-shot kills the norm, and limited movement options... new players can get into the game very quickly and contribute to a match, or get at least a couple kills.
UT2k* has suffered from a much steeper learning curve, and as a result... lower player counts. However, once you get a feel for the overall experience, I would say that the play is MUCH more rewarding and skillful.. and definitely worth the time put in (I make the checkers -vs- chess analogy).
Sadly, in today's age of instant-gratification... nobody wants to put time into getting good at anything anymore. If someone installs the game, steps into the Purgatory, and gets smoked... It's easier to put the game down, dismiss it as sucking, and move onto something easier.
edit: How the hell did that post get so long?.
