Why did capital ships adopt small-tube boilers slowly?

Why were small-tube boilers not adopted in the Queen Elizabeth class when related machinery was already available?

The technology existed, but the Royal Navy avoided stacking another major change onto a design already introducing new speed, armament, and gun layout.

Yarrow's boilers used progressively smaller tubes before the fully developed machinery normally meant by small-tube boilers became standard. Those earlier boilers should not all be treated as identical to the later type. Smaller-tube arrangements were already appearing in destroyers, cruisers, and battlecruisers, so availability alone did not explain the Queen Elizabeth choice. The explanation turns on a Royal Navy convention of limiting each class to three major changes. Higher speed, an eight-gun layout, and 15-inch weapons already supplied those changes, making another machinery innovation less attractive.