Most likely, when I took my post Queen Serenity gave me three taboos. The first taboo prohibits the travel through time by humans, the second taboo is that I am never to leave, and the third taboo is that I must never stop the flow of time; the ultimate taboo. As such to prevent a violation of the first knowledge of the guardian of the gates of time became forbidden, since I alone had the time keys.

[Until Small Lady stole one. Setsuna finished her tea, setting the cup to the side.]

Usagi is Princess Serenity and will one day be Neo Queen Serenity, but it was the previous Queen Serenity that assigned me to my post. And time has changed, the future has changed with all that has happened. Which is why I feel that we can change the fate of what is happening here as well.