1: Because many maps get removed from the whitelist and, as stated, most people don't know how to tell.
2: Even if it is whitelisted, there are a lot of maps that aren't.. well, good. Maybe they've gotten bad first impressions by Doom or something.
3: It's slightly too complicated to download the maps for most people, unless they're joining a server hosting it.
4: It's also more effort to host a server than most people are willing to put in, meaning they must find a public dedicated one with (or without) map voting and a rotation of custom maps.
5: There's, what, ~20 official maps now? How do you know they're just playing west london repeatedly? Sure, it used to be like that, and some servers run west london 24/7, but new players PROBABLY haven't even played it before. It's on the bottom of the maplist alphabetically too. Hell on Earth? A lot of the maps are simply extremely difficult, whether the difficulty is artificial or not. West London is a nice break.
6: Maybe people actually like the map. I can certainly see why, as it's one of the few made by Tripwire.
Personally I used to hate Offices when it was just one of five choices, but a while ago I started playing the more 'cramped' maps. Admittedly I think I learned every weapon and ammo spawn in a couple weeks of playing it rarely, but to me it IS almost new.
Camping the first floor is boring. Always.