Why Insect-class river gunboats carried 6-inch guns

Why did Insect-class river gunboats mount 6-inch guns?

The weapon combined useful shore-attack power and intimidation with the logistical advantage of an established Royal Navy calibre.

The Insect-class gunboats needed enough firepower to intimidate forces ashore and to strike targets beyond the riverbank. A 6-inch gun supplied that authority in a weapon already familiar to the Royal Navy. The choice also simplified ammunition supply compared with introducing a special heavy calibre for a small class. Their armament followed the demands of riverine presence and shore attack rather than a conventional fight against ocean-going warships.