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>>64485682
>textwalls
the concise answer is that submachine guns were considered secondary (even tertiary) combat arms to the infantry's primary issue rifle.
Not much engineering or doctrinal effort was given to them in particular. Mostly, they just needed to be low cost, accessible, durable, and operable with minimal/no training to troops.
Submachine guns of the WWII era were primarily expedient cheap weapons. There were exceptions, but most of them just weren't small arms of prime concern to the armies or generals/commanders in the field (though they appreciated having them around, and for specialized uses)