Because the market for a game like this is tiny on scale of things compared to the more mainstream PC and Console games. Most game publishers now aren't interested in publishing PC only titles it has to be multi platform as that's where the bucks are.
Even MS have pulled the longest running of them all "Flight Simulator". Market too small to justify cost for return etc compared to just pumping out some new XBox titles.
Plus another aspect for AH even though it's muti player and there have been some improvments over the years, the graphics look very dated. Even though the gameplay itself might be as good as it is, the eye candy for anyone new is first impression and compared to what any previous flight sim someone might be use to, trying this out it's like staring back into the 90s in some regards.
A Lot more games have bigger and better multi player support then they use to, the likes of ARMA2 and BF series adds more competition for anyones spare time and combat fix..
Basically for PC's market and sales there is WoW and Sims. Everything else is muti platform.
I commend HT team for all the work they have done, it's certainly been able to hold my interest off and on for the last 10 years.. But at moment that's leaning more towards off just due to my own commitments and free time nothing due to this or latest changes etc.. Though drop in numbers for MA during Aussie evenings is more noticeable lately than previous years or maybe it's just been a quiet few months, dunno.
Unfortunatly the market for this in it's current format is very small and always has been. Other then for when AW crowd joined I don't think the growth has ever really been there going by numbers in the MA's over the years. Always around the same.
<S>...-Gixer