I'm a fan of STEAM. I don't think AH should be on it.
A bunch of exposure isn't a good thing, unless it's the right kind of exposure and aimed at the right audience.
AH has a somewhat selective audience, that of WW2 aviation and history fanatics that want to fly simulators. The average uninitiated STEAM user woulndn't fit this bill.
Plus, Steam has to integrate all the games into their service. That would take it out of HTC's hands. This is one game that benefits from not having the slow-down of the STEAM middle-man.