Clearly, Strike Fighters 2 has not performed according to TK's expectations - by his own admission, SF2 North Atlantic still hasn't made back the money it cost to develop. There are a number of reasons why this could be so:
1. The game and its expansions are of poor quality.
I would dispute this, as I'm sure most people on this forum would.
2. Flight sims are an increasingly niche market, and there simply isn't enough interest in them to support decent sales.
I would dispute this as well. According to SteamSpy, the Il-2 games have hundreds of thousands of owners. The main DCS module has several millions. That module in itself is free, but it is at least useful as a tool to gauge the general interest in jet combat. If only a fraction of those people bought SF2, TK would be set for life. Obviously, those games were made by larger teams and their production values are well above what SF2 could aim for (although with the help of mods, it could certainly come close), but SF2 has other qualities that I am certain would appeal to a large portion of the owners of the games mentioned above.
3. Strike Fighters 2 is available for sale only to an extremely limited market, through an obscure website.
This, I believe, is why the game has failed to meet with financial success. It can only be purchased on the ThirdWire website, only a small portion of the gaming community even knows of its existence, and to make matters worse the game and its expansions are severely overpriced. Il-2: 1946, the complete original Il-2 package, can be bought for $10 on Steam, or even less during sales. SF2 Complete is sold for $100 and can only be found on the ThirdWire website.
It is all very frustrating, because I want SF2 to do well.