WHMB has pretty much nailed it. Boeing killed the 757 because airlines stopped buying it, and the 757-300 was a major flop.
The 757 had an interesting life cycle - originally planned to be ~150 seats, launch customers BA and Eastern convinced Boeing to up it to ~180 passengers which turned out to be too big, at least at EIS. It struggled for orders for several years after EIS and there were rumors that it would be cancelled when there was a major shift in airline strategy that turned the 757 into a hot seller through the 1990s. Unfortunately another shift in strategy after 9/11 turned the 757 into a dud (just as the -300 version was coming on-line). Soon the bean counters couldn't justify all that floor space to build one or two airplanes per month and it was cancelled.
The other aspect of the 757 demise was the 737-900 - it could carry a similar passenger load with similar fuel burn and operating costs, but cost the airlines a whole lot less to buy (there were several aspects of the 757 that made it relatively expensive to build). The 737-900ER may not quite have the legs to operate coast to coast 365 days a year, but the MAX should address that.
Ultimately Boeing built 1050 757s - which may pale compared to the 737 and A320 series, is still not that shabby. Market share was pretty close to 50-50 for the Rolls and Pratt engines - the difference being Rolls was spread over dozens of operators, while Pratt had a handful of operators that had a huge number of airplanes.