Even if people still play online there's no way you'll get ranked achieves. Ranked matches were dead when I last played over a year ago, all the action's in the player matches which has no ties to achievements. There's bound to be a boosting community out there if you really want them though. --- SMILE! :D
It's popular because pretty much in every game if there's a crappy map that is either: a) Easily exploited B) Requires little skill to get kills or C) is easily camped, it will be the most popular.
See: Nuketown in Black Ops.
One of the major reasons I stopped doing online gaming on PC was because after the first couple months a game comes out almost all servers play one map exclusively 24/7 (See: BF1942, BF:V, BF2, BF2142, CoD, DoD, CS, CS:S... you get the point) or some crappy custom map 24/7 (See: ALL variants of Ice in CS), and it's usually a bad one--and even if it's not a bad one, it just gets tiring to only play one. freaking. map.
Unforunately there's a heavy democratic aspect to the pre-matchmaking world (and even some post) where baddies get to decide to play only on maps they're "good" at.
Loved this game. Still think it's the most original CoD in the series aside from CoD for whose formula has now been exploited to hell and back. 24 player matches and vehicles? Damn CoD, you should have stuck the path, might have had a chance to knock out Battlefield in temrs of quality.