They did leave out the higher CR guardinals and archons, so along with leaving out the highest CR mondrons suggests a pattern - that we'll be seeing them all in a later product (I'm not sure of the rilmani hierarchy, so I'm not sure if the pattern holds up there). As
@Parmandur has said, they're wanting some higher CR creatures for the '24 MM, and the ones missing here would fit nicely, especially the celestials. Beyond that, there are still plenty of Archdevils yet to get 5e stat blocks, so that's somewhere else we might see these.