I would guess it is a mix of factors. The 21-inch torpedo thing really reached back before NATO.
The general size of "close to 21 inches" (for a while the Germans played with 50cm while French torps tended to be 55cm until 1964) was probably an unspoken consensus of being the best compromise between capability and portability (which also means number of rounds carried), especially inside the confined spaces of a sub.
When combined with the influence of the Imperial measuring system, 21 inches have become the norm more than not. Then of course NATO futher standardized things.
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