Why battleship guns larger than 18 inches lose value

Would 20-inch or 21-inch guns be worthwhile on a battleship?

Their loading and maintenance burden would outrun the useful increase in firepower; only a much larger jump might justify the slower rate of fire.

Weapons of 20 or 21 inches occupy an awkward middle ground. They are much harder to load and maintain than 18-inch guns without enough additional effect to justify the penalty. A 24-inch weapon might offer a clearer increase in hitting power, but its rate of fire would be extremely slow and interwar technology would make it largely a prestige weapon. An 18-inch treaty ceiling could also tempt Japan toward an enormous 20- or 24-inch-gunned ship that required new construction infrastructure and consumed resources sufficient for several smaller hulls. Calibre has to be judged through the complete ammunition-handling, industrial, and firing cycle, not shell weight alone.