But if you go back to Wonder Woman's first appearances in All Star Comics #8 and Sensation Comics #1 and actually look, it's clear in many panels that the two legs of the "skirt" are separated, and in fact it's a pair of loose culottes.
Over the following issues the culottes become tighter and more streamlined (shrunk in the wash?) until by issue #9 they are the Familiar star-spangled shorts associated with Golden and Silver Age Wonder Woman.
But they were never a skirt.
Images from Sensation Comics #1.