With the introduction of cross-realm trading, it’s now possible to access items on the AH of any server, it just requires jumping through hoops. You either need 2 accounts yourself, or to have an intermediary who helps you trade things back and forth. Based on that, it seems like there’s no great reason to still keep any part of the AH limited to one server(cluster), and with consumables/mats already being region-wide, presumably the tech already exists but has been restricted on purpose.
Mail being fully region-wide just seems like a natural extension of the above, so thought I’d throw it in here as well.