A response of "…. Why did I never wonder about that? Why don't I know?" would be a great way to introduce a quest to find the answer, but I suspect you've already given our heroes enough to do.
My favourite answer to the last question is that when Usagi transformed for the first time, that action set the base pattern for all modern-day Sol-system Senshi. (They aren't duplicating Sailor V because that isn't Minako's "proper" transformation.) The magic that set the costume did so by modifying a uniform Usagi already wore, investing it with power and authority. (If there were any other male "Senshi", they would presumably wind up with a variation on Tuxedo Mask's outfit; it can be concluded that Mamoru attends a lot of "tuxedo required" events.) Yes, this is a Shadowjack theory.
My favourite answer to the last question is that when Usagi transformed for the first time, that action set the base pattern for all modern-day Sol-system Senshi. (They aren't duplicating Sailor V because that isn't Minako's "proper" transformation.) The magic that set the costume did so by modifying a uniform Usagi already wore, investing it with power and authority. (If there were any other male "Senshi", they would presumably wind up with a variation on Tuxedo Mask's outfit; it can be concluded that Mamoru attends a lot of "tuxedo required" events.) Yes, this is a Shadowjack theory.