I always feel like this game remains in the shadows because that's where it is comfortable. This game required a large degree of effort to get over the learning curve (for me playing alone, anyway) and once you understand basic gameplay principles, there's still significantly more to learn. I'm barely ending my first year of ownership for ARMA II and yet I still play on recruit difficulty. (That might just be my absolute incompetence in combat).
Although I have barely played any real multiplayer, I have played several games with my friend, and the excitement that builds is quite a thrill. In short, ARMA II is a game which affords the player complete and utter freedom, once they've hurdled the learning curve. I think the learning curve itself is all the initial player sees, and they feel that it is representative of the rest of the game. Therefore, anyone who attempts the demo initially might be inclined to reject the game, since their initial proficiency with it is sub par.
Finally, I think a large group of younger audiences lose confidence when faced with the minority of people who are openly hostile against those under the age of 16 playing ARMA II. That may contribute to me playing with a single person constantly.
So I consider the primary lack of popularity of ARMA II to be based in its reclusive nature, steep learning curve, and the seemingly harsh community. (Although after you're in the community, you realize how they actually are).
All of this was probably covered earlier, but this is my 2 cents. And I'm not fond of reading 147 pages of posts.
EDIT: Sorry for posting in this old thread. I forgot to look at the latest date. My first post too!