When I came back 10ish years ago I was forced to play GOWF because nobody played anything else. Before that I NEVER played GOW and hated it because that is all anyone used to play back on BNET. At that point, there were a lot more people playing on RU. So, I don't think the lack of diversified maps has anything to do with it.
As SG basically said, people are getting old, the game is old, and younger people don't want to play this given the other choices out there. We can give it the ole college try in an attempt to revive it but let's face it, it is what it is. Enjoy it while you can.