Reliability decided early 4.5-inch mount choices

Why was the upper-deck 4.5-inch mount not used more widely on earlier ships?

The early upper-deck mount had balance and maintenance issues, so the Royal Navy kept using the more reliable between-decks pattern until the design improved.

Between-decks and upper-deck mount layouts involved more than a simple difference in quality. The Mark III upper-deck mount proved troublesome in balance, bearing loads, and maintenance, so the Royal Navy continued with the more reliable between-decks arrangement. An upper-deck mount was generally preferable away from capital ships once the mechanism worked reliably. Later Daring-class and Australian Battle-class installations show the mature solution.