Well the other 2 were made by IS and Blueprint... not exactly the same level as an FSDT or FlyTampa. Though, I'm rather curious as to what the reasoning was to ditch Spokane. Commuter flights between SEA, OAK, SFO, PDX, SAN, LAX, etc would seem to have put it on a moderate return-on-investment.
If I remember correctly, Buffalo didn't sell nearly as well as George had hoped. That is in reasonable proximity to numerous major airports as well. Something tells me that had he done Spokane, it would be much like Buffalo. But I'm guessing there is more to his decision than that.
Perhaps a little more research into who-fly's-where would be a good thing. I rarely fly on the East Coast, and though I have Buffalo, seldom do I fly there because Niagara Falls used to bog my system down. I suppose it would be a good idea to poll those of us as to where we fly.
With the advent of Orbx and smaller airports on the West Coast, there is a draw to scenic areas. This may be a large stake in why Buffalo was not as successful; there was only one major airport from a well-known designer/publisher in close proximity
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In other words, there wasn't enough airports around it (say within 250-300 nm) to help support it. We just had JFK to fly to and from Buffalo. There are some great Canadian payware airports near Buffalo, but they were too close. That's why GEG makes more sense; as it supports commuter aircraft, and larger continental traffic. I think that's also why SAN was a success. LAX, LAS, SFO are rather nearby, and SAN also handles inter-continental traffic (and international traffic, for that matter). Plus, SAN also has an amazing approach into 27... and people really like a challenging, interesting approach. Just look at Kai Tak. It had been out of commission for years, but the FS community really supported it when FT did it.
So, that's my theory on airport development/placement - however, I'm sure the folks at FT, and especially Virtuali have taken a hard look at the numbers of sales, in addition to what they think would be a fun place to fly.