Are you asking why the 757 was pulled from production?
The reason why is quite boring and straightforward but the results are juicy like watermelon :).
Why pulled from production?: Order book was drying up and the 757-300 didn’t have enough of a future to justify keeping the line open. The cleared factory space could be put to better use (Speed up 737 line for example).
Result: Boeing pulled the plane shortly before airlines needed replacement aircrafts for their oldest 757s. Buying smaller but still capable and more fuel efficient aircraft (737-900ER, A321-200) was a good stopgap. But you can only replace so many routes with those 2 airplanes*. Airlines wanted the 757s superior range, takeoff performance, and lifting capacity (Pax and cargo). Boeing could not offer that so airlines now are forced to purchase the A321neoLR, upgrade some 757-200 routes to the 767-300ER, or use smaller but similarly capable aircraft like the A319 for some routes.
Note: I know that other planes replaced 752s other than the 739 and A321