I am a 757 pilot, and I love the 757. But, from the point-of-view of the airlines, the 757 is a pig. It’s an overpowered monster that was built for the specific purpose of getting into and out of short, high airports. With the advent of longer runways and better airports, the 757’s purpose no longer exists…

The 757 was specifically designed to replace the 727, an airplane that was designed to transport 200 passengers in and out of rough, short, high-altitude airstrips. That mission no longer exists. It’s a bulldog, built for an earlier time when “sports car” airplanes were a necessity.

Today’s modern airports only need “air busses” that can handle a large number of passengers on long concrete runways. Hence, the stretched 737s and A321s.

I am currently stationed in Kona, Hawaii. Delta flies a 757 in and out of this airport once a day, to/from Los Angeles. The airplane only uses the northern half of the runway for takeoff and landing, about 4000–5000 feet, and climbs out like a homesick angel. United’s 737 to SFO routinely uses 9000 feet of runway for takeoff.

Here’s a 757 departing Kona for the 5.5 hour flight to Los Angeles. The taxiway near where the airplane lifts off is 4500 feet from the approach end. Watch that climb!

With all due respect to the previous answers there is some information listed here that is kind of true, but not the actual reason. The 737 is a reliable old workhorse of a design. Anytime you design an new “white sheet” airliner from the ground up it will need to go through a lengthy and expensive “type design certification” from the FAA and other international governing bodies around the globe.

However, if you do a “stretch” but maintain the key elements such as aircraft handling and pilot “feel” for flight controls, keep the same safety features, ect… you will not have to do extensive “type design” certification and all new pilot training.

This will also, allow the maintenance manuals and training to be identical to the current type. If you are a customer this a big plus as you do not need to retrain pilots or maintenance personnel. This can save an airline tens of millions of dollars in additional costs.

Interestingly, it was this exact reason that Boeing developed the “Maneuvering Characteristics Augmentation System” (MCAS) that caused the two fatal crashes of the new 737-Max airliner. The MCAS system was developed because the larger more powerful CFM LEAP-1B engines used on the 737-Max are simply too large to fit beneath the wings of the 737 airframe.

The original 737 Type Design was done back when High Bypass Ratio jet engines were not yet being used in commercial aviation and therefore, the smaller diameter of the original design lead to the plane not needing the ground clearance of the much larger diameter modern engines.

Boeing’s fix for this with the 737-MAX was to push the engines further forward on the wing mounts in order to clear the ground. This led to a change in felt pilot handling characteristics of the 737-MAX relative to the previous 737’s.

In order to not have to do a new type design certification the MCAS was implemented to make the reconfigured 737MAX “artificially” feel the exact same as any other 737.

This unfortunately led to a tragic result as not having to retrain pilots for the 737-MAX, left many pilots not fully understanding what the MCAS was doing on its own to adjust the planes trim during flight, and how to respond properly to an angle of attack or speed indication malfunction due to a bad AOA sensor. This simple work around a re-certification and training led to tragic crashes.