I'm just curious, like which ones?
I have been playing Aces High since the first weeks of the Beta.
I have spent 15 years playing the Battlefield series.
The majority of my gaming has been those two games for the last 20 years.
In the whole, compared something like Battlefield 4, Aces High is no more complex, IMHO. Different skills and knowledge, but I've played both, it's not more complex.
The issue is that a lot less players have played other combat flight sims, and so many more players have played one version of FPS or another. So it is a lot easier for many to jump in BF4 and play well enough to at least have fun, than it is for many to jump into Aces High and enjoy it enough to stay past two weeks. It's harder for many to get up on the initial curve to start see the progress it takes to motivate them. However, to master both games and be a top competitor, takes about the same amount of skill and effort.
So that would explain why we have a hard time converting new players who come from a FPS or mouse flying background. It doesn't explain why we aren't converting the millions of IL2 players. The learning curve must not be the fundamental issue there.