Although Tsukuyomi's "Full Moon" and "Waning Moon" Divine Bones are somewhat similar to a certain extent, for ordinary deities, it's challenging to simultaneously accept and utilize these two Divine Bones.
Take Kaguya for instance.
As Princess Kaguya mentioned, if during that "accident" on the dark side of the moon more than a thousand years ago, Tsukuyomi's Divine Bones were kept and held separately by Kaguya and another deity, then the situation at that time might have been different.
Perhaps the "Waning Moon" Divine Remains Bone might not have been tainted.
And now, Princess Kaguya, in terms of Divine Power, definitely cannot compare to Tsukuyomi, and not even to Kaguya at her prime.
Therefore, finding another deity to accept the "Full Moon" Remains Bone has become very necessary.
This is also just to safeguard the Lunar Palace.
And Kamiyagawa's Dream Devouring Tapir is undoubtedly suitable for Tsukuyomi's Remains Bone.
