Looking up other airframe manufacturers I realised that the B767 was introduced prior to the B757. And that other airplanes featured slide-up doors as well but the B767 was the last one featuring them. Could it mean it was some kind of fashion of the airplane design until the late 70s?
Outward swinging doors require a great deal of effort to open and close, and can require the operator to potentially lean outside of the aircraft and risk a fall to close. The 757 door originally had an issue where shorter operators could not get enough leverage to open (see: Making It Fly in the Seattle Times. Upward doors were used for a while but they are finicky to operate and maintain and if the opening mechanism fails can be very difficult to open in an emergency.
The translating "gooseneck" door was originally designed by Airbus. The design is simple, reliable and easy to operate. It has more or less become standard on all new aircraft designs.