Having already instructed Moshe (in Pasuk 16) to place the Luchos inside the Aron, why does it repeat it here?
Ramban: It is coming to explain the role of the Keruvim in the Mishkan - to form a Throne of Glory for Hashem, 3 whose Shechinah will descend to between the Keruvim, from which point He will communicate with Moshe. 4
See Ramban's objection to this explanation. Moreover, in the earlier Pesukim, the Torah only mentioned the lid after instructing Moshe to place the Luchos inside the Aron, in which case the placing of the Luchos preceding that of the lid is self-evident?
Which explains why Hashem is called Yoshev ha'Keruvim", Shmu'el 1, 4:4.