Another important point is how many airlines have standardized their narrow-body fleets. Maintaining a single type/family is simpler and often cheaper than havi

Another important point is how many airlines have standardized their narrow-body fleets. Maintaining a single type/family is simpler and often cheaper than having trained crew, mechanics and parts available for multiple types of aircraft. In the US, for example, Southwest and (mainline) Alaska are both pure-737 fleets. Virgin America, Frontier and Spirit are all A320-family, with JetBlue operating only A320 family and the E190. US Airways, prior to its merger with American, was moving toward an all-A320-family narrow-body fleet and was nearly there (only a few 757s were left, and US Airways was looking to replace them with a long-range variant of the A321 once it became available). So a new narrow-body type -- whether it's COMAC's C919 or Bombardier's C-Series -- probably will never make it into those carriers' fleets, which in turn makes it that much harder to break into the market. Added to that, the existing manufacturers (Airbus and Boeing) are established enough and have the resources to be able to offer significant discounts to an airline which commits to a large fleet overhaul. New entrants likely are not financially able to do that, putting them at more of a disadvantage in trying to place their first aircraft with the airlines.