My theory:
TOEI will let you do anything you want.
I, quite frankly, was surprised that S was dubbed in English to begin with. I think that making Uranus and Neptune "cousins" made the situation and the context of the season even more awkward.
My opinion: Stars is just too queer for audiences in the United States to handle, and, therefore, it cannot be aired in the United States. Kindly notice that I am *not* using the word queer pejoratively. The word is associated today with empowerment. So Stars... well, Stars depicted two lesbians raising a baby and it also had characters able to change their gender from male to female. This is shocking for audiences to handle... even today.
If a TV station in the U.S. decided to air "Sailor Moon S" and "Stars," uncensored, as your typical Saturday-morning cartoon, today, I feel that tons of conservatives would feel outraged. Organizations like one-million moms would throw a fit over the inclusion of lesbian and queer characters and launch a massive protest on social media to get the show pulled out. If things are like that today, just imagine how things were back in the late-90s/early-00s when S was being dubbed.
I do not think the U.S. is ready for a season like "Stars," and, in my opinion, that's the *real* reason why Stars didn't get its English dub.