Why should they retired sets if they are selling good?
For example, the modular house parisian came out 2015 and retired this year. Other modular houses have come out after that and retired earlier.
If other themes sells worse than hp they will retired and that will create shelfspace for hp.
I have seen great hall, whomping willow, Hogwarts express like "best seller" in some websites in my country. But I dont know if the smaller sets sells good.