I think it's mainly because of marketing. For example, AA may have a flight at the same time from BNA to ORD with a connection to IAD. If NW however markets a direct BNA-IAD flight with a stopover in DTW, it will go to the top of the heap of the computer display in a travel agency. That was what a travel agent explained to me some years ago. I don't know if this still holds true and I also don't know if the same theory applies to internet sites such as Orbitz et. al., but it made sense when it was explained to me at the time. Now as far as the plane changes are concerned, I think years ago they tried to keep the same aircraft on the entire itinerary, but I believe now the name of the game is matching equipment to demand.