Why Emerald-class cruisers entered service with triple tubes

Why did the Emerald class initially receive triple rather than quadruple torpedo tubes?

The triple mounts were available immediately; commissioning a capable cruiser was preferable to waiting for the desired quadruple tubes.

The priority was getting a useful cruiser into service rather than holding it idle for a preferred mount. Triple tubes were already available and workable when the quadruple mounts were not. Using two triple sets per side sacrificed only two tubes from each broadside compared with the intended fit. The Royal Navy could therefore commission the ship promptly and replace the mounts later instead of allowing one unfinished weapon system to delay the entire cruiser.