While I'm not sure of the official reason(if there is one, apart from maybe economics) I did read an article early this year in which it was mentioned that western built aircraft were being acquired and placed on important routes to try and win business class passenegers. Aeroflot felt that they were being by-passed by the important business paxs because of their Russian built aircraft, so B737's, B767's and B777's as well as the A310 (to a lessor extent) were placed on routes to America, Germay, England etc..
Now I know Russian aircraft do these routes as well, and that certain russian aircraft are just as well built etc...
but looking at it from ecomonic viewpoint, you have to see where Aeroflot are coming from
Hops this maybe helps a bit???
Aircraft I've flown: A300/A310/A320/A321/A330/A340/B727/B732/B733/B734/B735/B738/B741/B742/B744/DC10/MD80/IL62/Bae146/AR