I always figured people choose them because they're easy to win. I've kind of found it with all servers, really...
West London, Mountainpass and Biotics Lab seem to be the ones I see the most for servers with default maps only. In each of these you can see the enemy a mile away so you know what they are, in Biotics it's a bit similar - But since it's corridors, they're pretty much bottle necked everywhere.
In maps with custom maps, it's always Arcade Gas Station, Doom and Basin sands. Doom is similar to Biotics Lab with its corridors, Basin is Mountainpass. And for Arcade Gas station, it's just pure easy; Get two people to weld the doors and the people on the roof just shoot them dead.
If ever I complain to my team about how everyone chooses the same stuff, I find that they wind up picking something similarly easy; Such as Colosseum. Another wide open map to see things in every direction.