>>152454
money. due to fuel costs trijets and quadjets like the md-11, dc-10, and 747 fell out of fashion. pilot costs are also another factor, easier to have just a few planes than many. also airlines opted for frequency after 9/11 which led to popular routes like LAX-SFO and LAX-LAS to be serviced by 20 E175s per day instead of like 3 767s or 4 757s.