I recently told some of my coworkers about AH (all of them are gamers), and they all claimed to have heard of it before. Why not play it?
I'm guessing a WW2 air combat sim (it does come off as "air" to those who've not investigated further) has limited appeal in this age of X-boxers. Add to that the cost of stick (throttle, peds) a computer capable of running the graphics, etc. Then there's the learning curve with very little instructional guidance for the beginner. The economy is not exactly robust and many folks are dialing back on expenses.
The older days of H2H allowed new players to develop (and yes, there were freeloaders and hackers) skills before flying the arenas.
No doubt, HTC is tinkering with ways to make the game attractive to more folks, e.g. the new series of films Skuzzy & Co are working on.