It does seem to be a fleece type thing, with down-type sheep getting it most ie dense, fine coats, fibres all the same length. We have had sheep like that and there's not a lot you can do except house them, but that has its own problems.
I can be smug
because Hebrideans have a shake-dry fleece which also naturally sheds water as it falls, so what you lose in fineness of wool you gain in resistance to rot. Heb fleeces do have a bit of a tendency to felt in warm weather with heavy rain if you don't get the fleeces off pronto in June though.