Why so many navies adopted the OTO 76

Why have relatively few navies adopted the OTO 76?

They have not: adoption is widespread, and Canada's use on Cold War Tribal-class destroyers shows how reliability in demanding conditions helped the gun earn a place in many fleets.

The OTO 76 is widespread enough that a modern navy without it is more unusual than one that uses it somewhere in the fleet. Canada selected the weapon for its later Cold War Tribal-class destroyers. Their northern operating environment made reliable performance in freezing conditions especially valuable. A navy can retain it as an important fleet capability without fitting the gun to every hull.