Why US pre-dreadnought gun calibers varied

Why did US pre-dreadnought classes use several different main-gun calibers?

Designers used the best guns available at each point, while Congress repeatedly shaped the resulting programs.

American pre-dreadnoughts used the best guns available when their classes were designed. Congressional influence also shaped the choices. The resulting sequence therefore reflects both technical opportunity and the political system through which the navy obtained ships. It cannot be read solely as a succession of preferred naval calibres.