Why the Admiral-class hulls made better carriers

What prevented the original Admiral-class battlecruiser scheme from working?

Its speed requirement repeatedly destabilized the armor arrangement, making aircraft-carrier conversion a more useful employment of the large hulls.

The planned battery itself was not the central problem. Designers kept moving protection and internal arrangements while trying to preserve the required speed. Those interacting demands prevented a stable, convincing battlecruiser solution. Completing more of the large hulls as aircraft carriers would probably have extracted greater wartime value from the investment.