Why riverine gunboats need a separate comparison

Why not include Soviet riverine gunboats with T-34 tank turrets?

They came from a separate design tradition. Including them would require a wider comparison covering equivalent river gunboats, including the Royal Navy's Insect class.

Soviet riverine gunboats with non-powered 76 mm T-34 turrets overlapped with coastal gunboats in some tasks. Their design philosophy and development differed from motor torpedo boats adapted for gun combat. A comparison broad enough to include them would also need equivalent craft from other navies. The Royal Navy's Insect class illustrates how much wider that subject would become.