Dont forget the larger regional jets!
The 732 and 735, when introduced, were usually an airlines smallest jet airliner, used for short frequent routes or medium range thin routes.....those routes today are dominated by CRJs and ERJs in their various sizes, and with the larger RJ
models now coming on line, airlines are chosing regional jets to fly these types of routes more and more due to lower costs. (Think union rules.) Not only has the 736 sold rather poorly in this new enviornment, the 717 has had problems finding its market and A318 sales have also not been brisk.
The other issue with the 736 (and A318 for that matter) is that these airliners have "too much" capability and too much range, making them heavy and not as economical as the airlines would like them to be. The 736 can fly US transcon without a problem, something that the 732 could never do, but the price is rather average economics on a short-haul run. The 736 is a super aircraft for longer thin routes, like Hartford to Los Angeles or Philadelphia to Portland or New Orleans to Seattle, but in our world of mega-hubs, these routes have simply not developed. Thus, the 736 remains an unpopular member of the successful 737NG family.