To be honest, I often feel like maps in competitive games become popular for seemingly no reason at all. As a long-time CS player, dust and dust2 have always felt rather stale to me. I've always found the dynamic, multi-level combat of maps like assault and office to be more fun. Fortunately, I've been hearing a lot more people piping up in favor of traditionally unbalanced maps like nuke these days!

(What's annoying is when people try to attribute a map's popularity to its design. I've heard some people say that fy_iceworld is a brilliantly designed map. Seriously.)