Just guessing one of the following~
a. New eye candy would cull out a number of present players with older systems, like me.
b. HTC with limited programmers priorize other areas...plane updates, etc.
c. Most players are fine with what we have now.
If you REALLY want an answer to your question, call 817.431.8840...ask for Dale. 
my .02
a. AH can be run on an ati 9700, even with some basic eye candy on - thats a 2002 video card. An about 50$ hd-4670 or a 8800gt actually handles this game quite well - only the shadows and the reflections need more, but much more GPU power.
b. exactly what you said.
c. true, but its sad, if you think about it. The new planes are beautiful, the flight modelling is great, but i personally didnt like the landscape. This could be esily improved, just look at USRanger's works, what a difference that is. Using his textures in the MA wouldnt slow down the game too much - at least i think so. True, the terrain files would be much larger, for sure.