Boeing stopped production of the 757 for the very good reason that airlines stopped buying it. The last 50 built were spread over three years, early 2002 to early 2005, that's less than 1.5 a month, and the space at Renton was more worthwhile building 737NGs, which were in high demand. The last incarnation of the 757, the series -300, had particularly flopped commercially.
The airlines stopped buying it because it was expensive on fuel per seat-mile.
When the aircraft was first designed by Boeing it was a joint project with the 767. Boeing's initial hope was the 757 would replace the 737, and the 767-200 replace the 727-200. This seemed excessive, and turned out so. However, for some time it was not used as a US transcontinental aircraft, this came much later, and the scheduled transatlantic flights later still, although it did some pioneering transatlantic charter operations for European holiday flight operators over to Orlando and the Caribbean, stopping at Gander or Bangor to refuel. But the two major pioneer operators, Eastern and British Airways, used it on decidedly short/middle range operations.