Why the Queen Elizabeths lacked small-tube boilers

Why did the Queen Elizabeth class not receive small-tube boilers?

Jellicoe accepted shipyard warnings that the unfamiliar technology would cause major delivery delays.

The decision was driven by programme risk rather than ignorance of the newer machinery. Shipyard owners warned John Jellicoe that small-tube boilers were still unfamiliar in this application and would cause major delays. He accepted that advice because the Queen Elizabeths were required on a demanding construction timetable. Schedule confidence can defeat a technically attractive improvement in a major procurement programme.