It's got a large user base in this day and age when everybody and their mom's releasing a game and players are spread thin. Some people buy it just for that. Of course a huge user base means all kinds of riff raffs, hacks, and the like.
It's got momentum, and that comes from years of weak competition. It has the full weight of an entire publisher and its army of marketing monkeys behind it. People think they're buying for quality when they're actually buying for brand valve.
The right kind of shooter will come out and knock it off its perch. It gets easier as more iterations are released and they get more and more arrogant. Either way it's a fight for the crown of the summer blockbuster, whereas the game itself is nothing to boast about.