There are several main reasons. Boeing 737 has a long history. The world's first B737 was delivered on1February 28th, 967. The B737 is undoubtedly one of the most successful models in the history of civil aviation. It is favored by airlines, especially low-cost airlines and small airlines (please note here that this sentence does not mean that major airlines do not like it. At present, major airlines in the world still have large B737 series aircraft, but compared with the expensive price of Airbus, B737 series is obviously more suitable for small airlines and low-cost airlines. Therefore, B737 sold about 1.3 million aircraft, making it the largest number of trunk passenger aircraft in the history of civil aviation. Boeing saw the success of B737, and made a lot of efforts on it, constantly improving and strengthening to keep up with the times. It is more in line with the taste of airlines. From the initial100,200 to the most NG series and the "famous" MAX series, B737 has come out with 14 model.