Ask any PC flight-simmer and I'd bet 99.98% of them have played or know of IL2 and it's spin-offs.
I'd bet that only about 5% have ever heard about Aces High. Maybe only 2% have ever subscribed.
Il2 might have an edge in graphic effects (with the offsetting performance load), it has a nice damage model, etc, but I'd argue that the difference in exposure is almost totally due to the fact that is has a stand-alone single-player campaign/mission mode.
I believe there are several orders of magnitude more regular Joe's out there that just want a game they can fire up offline and play for an hour and pretend they are a WWII fighter pilot. They might have an hour between putting the kids to bed and calling it a night because they have work tomorrow. Most regular Joe's have no desire to put the time and effort it requires be competitive online against other players.
They may dabble in it occasionally if it comes included in the game they bought, but they are not going to pay a monthly fee to compete against people who spend every waking hour obsessed with the game.
I'd love to see HTC be able to access some of that other market someday. (In addition, not instead of.) If nothing else, it could provide a vector to increase visibility of Aces High.
$0.02.

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CptTrips