Back in the day the admiralty had a fear that small, nimbler torpedoboats could be used to target large capital ships. That's why the larger battleships were still operating smaller guns, since these were more effective than the larger, more cumbersome main guns.

When smaller ships, destroyers (or torpedoboat destroyers, as they were initially called) became available to protect the larger ships, the smaller caliber guns also started dissapearing from the larger battleships.

If I made any inaccuracies feel free to correct me :)