I think a lot of this will have involved the consideration that uptake of new versions is slow.
The Sim is so vast that upgrading to a new version (O / S Issues aside) is not alway's desirable.
With so many different scenarios available, it would take years to go through them all. Added to this is the slightly different "Feel" each new version brings. I must admit that the current version gives me plenty to go at, I know the controls and all comms. options off by heart, and would not feel the need to fork out £££ for an new version which may only add a few aircraft and result in a learning curve to find all those "Tweaks" that I have setup just right at present.
I wonder how much this is to do with profit or the lack of it? Game developers seem to be having a rather hard time of it at the moment with THQ announcing a $166,000000 loss over the christmas period. EA have announced an even greater loss over the same period. I believe that only two out of ten developers ever see any profit at all which is a bit of a worry for the future supply of games coming our way.