@GenericArchangel
Narratively speaking, I figure that it’s likely because Cheerilee was in the show for the purpose of being the teacher figure in the CMC’s stories – her roles have always been limited to stories dealing with school and schoolyard plots.
Thing is, there really haven’t been a whole lot of school stories anymore – the CMC have largely moved past that point by now. I’m of the opinion that “Crusaders of the Lost Mark” marked (heh) a fairly important transition period for the CMC – their getting their cutie marks, at they very least, coincides with them moving on from the stories and trials of childhood and into the stories and trials associated with being members of a larger community with more clearly defined roles and places within it. Their models haven’t changed beyond the obvious, but insofar as the show’s stories are concerned the CMC have very much stopped being kids and started being young women.
As a result, while it’s still mentioned that they got to school and such, the older stories that involved them being students and kids and dealing with the things kids deal with just don’t happen anymore – all their recent stories had them as teachers or counselors or camp directors; always the ones directing things and in charge and teaching, rather than the ones being taught and directed and aided by their elders. The show doesn’t visit their old school much anymore, and this means that there just aren’t many stories where Cheerilee has a place in or role to play for.
This is also, I think, why we haven’t seen Diamond Tiara or Silver Spoon much anymore. Their roles were the be schoolyard rivals and playground bullies; the foes of children’s personal stories. This sort of antagonists doesn’t have a place in the stories the CMC inhabit now – it doesn’t make much sense for people given enough respect by their community to do what the CMC do now to be seriously hindered by a petty bully and her yes-woman. They were redeemed, yes, but that brings in another problem – there isn’t really a narrative role that they could play as allies or friends, either; the CMC never had any real need of recurring friendly characters before, and they don’t really have such a need now. There wasn’t any empty narrative hole that DT and SS could move into post-redemption. The CMC’s newer episodes don’t demonstrate any need for addition characters in their narratives – they work just fine with the trio and whichever one-shot character is needed for this specific story.
TL:DR: Cheerilee, Diamond Tiara and Silver Spoon don’t appear anymore because the Teacher, the Bully and the Toady are archetypes endemic to stories about children and school, and the show doesn’t do those stories for the CMC anymore; and because they aren’t needed for other roles in the stories written for the CMC now.