Because they only sold just over 1000 of them the first time round. It was hardly a success.
Well, air traffic more then tripled during the last 20 yrs and the A300/A310/767 are also getting old. An entirely different situation.
It seems the >200 seat segment wasn't lost on Boeings agenda after the NSA cancellation / MAX launch.
Boeing confirms long-haul 757 replacement study