Why 21-inch torpedoes became the practical standard

Why did torpedoes largely standardize around 21 inches?

The 21-inch size offered a workable balance of effect, weight, storage, reload capacity, supply, and compatibility with existing platforms.

Larger diameters could offer advantages, and the Japanese pursued the 24-inch Long Lance, but growth in torpedo size imposed steep penalties. A 21-inch weapon was easier to carry in useful numbers aboard destroyers and to store aboard supply ships. Standardization also protected the value of existing tubes and ships: changing caliber could demand an expensive rebuild of the fleet's supporting infrastructure. The answer frames caliber as a force-wide logistics decision rather than a simple question of maximum warhead size.