Because: what's on and in the box would silence the naysayers and doubters.
The attonement(hasheva) piece with
it's arch guardian On top and HaShevet (staff) inside the lid.
Numbers/Korach
17:25 Vayomer Adonay el-Moshe HASHEV et-mateh Aharon lifney ha'edut lemishmeret le'ot
livney-Meri utechal telunotam me'alay velo yamutu.
God said to Moses, "Put Aaron's staff back there before
the [Ark of] Testimony as a keepsake. Let it be a sign for anyone who wants to rebel.
This should put an end to their complaints to Me, and then they will not die."
This validates my previous points in my recent thread, where knowing Hebrew and acknowledging our translations of names into definitions or vice versa definitiones made into personas (names) is problematic, often missing the simplest of messages.