Before girls swimming suits, boys always swam in the buff, for centuries upon centuries. Such a rule in the depression years would have made the pool accessible to the poor who couldn't afford swim suits, or who's underwear might have well caused them ridicule. The question is not really why they did not allow swimsuits, but why you did know.
But why did I know what? What about before the depression years? How about in the 13th century, could people afford swim wear then? I take it you never took Dulcinea swimming. Your post really makes no sense...........