Why Bofors did not replace the pom-pom before 1939

Why did the Royal Navy reject the 40 mm Bofors before World War II?

It did not reject it outright; existing pom-pom production and late availability governed the transition.

The Royal Navy already had substantial pom-pom stocks and production when newer anti-aircraft weapons were being introduced. Bofors and Oerlikon adoption depended on time and funding. The Ten Year Rule had delayed procurement and encouraged planning for a war from 1942 onward. Replacement weapons had been identified, but money had to be allocated among competing needs before the expected war arrived.