The "other Earths" mystery is in practice an ongoing arc in TOS, if you want to look at it like that.
Early on, our heroes are stunned that such things could exist, but can do nothing beyond, well, being stunned. After they report home, this guy or gal Hodgkin formulates a theory of parallel development to try and make sense of it. And then the heroes uncover plenty of evidence that the galactic deck is in fact getting shuffled big time by greater forces: by powerful ancient cultures that seed and transplant, by time and space machines that whisk meddlers to places that should have nothing to do with Earth influences by virtue of their spatiotemporal coordinates, and by forces that craft or move entire planets either through advanced technology, or then simply at will.
By the end of these hectic five years at where apparently none indeed had gone before, the heroes should have resigned to the fact that the Milky Way is littered with Second Earths, and no doubt Second Vulcans and Second Andors as well. They need not have discovered why this would be, but they would have uncovered several routes to how this could be. And there really shouldn't be all that much fuss about it any longer.
Timo Saloniemi