The general consensus among the flight sim community is that their license from Microsoft forbade them selling it as a video game, to avoid them competing with first Microsoft and later Dovetail.

There's actually a reason for that: Microsoft had a separate product called ESP [1] that was based on the Flight Simulator codebase that was marketed as a platform for doing industrial training simulations.

Development stopped with the dissolution of ACES, the Flight Simulator team, but Lockheed acquired rights to continue to develop MS ESP, which eventually became Lockheed's Prepare3d [2].

[1] https://msdn.microsoft.com/en-us/library/cc526948.aspx

[2] https://en.wikipedia.org/wiki/Microsoft_Flight_Simulator#Clo...

Licensing is weird sometimes.