The forest moon of Endor is sometimes referred to as "the Sanctuary Moon".
Is the term "Sanctuary Moon" a proper name, unique to the Ewok's homeworld, or is the forest moon of Endor merely one of many "sanctuary moons"?
Endor was set aside as a protected nature reserve during the Galactic Republic, giving it the name Sanctuary Moon. - Endor | Wookieepedia
Unfortunately, this was decanonized and there is no other reason mentioned.
That said, it is a reference to The Forest Moon of Endor. Much in the same way New York city is also called The Big Apple. Even in Legends, this appears to be the only place by the name of "Sanctuary Moon".