Because if the current prototype works sufficiently it will be a lot better to just use that design everywhere. And it does stop them. The first Everchosen at head of massive army only showed up after Waystone network lost its protection, Skaven reared their ugly head and Nagash poisoned Nehekharra. Why do you think that is?
The current prototype is a showpiece that is limited by the number of Arch-mages in Laurelorn, more than that by the number of Arch-mages willing to do boring repetitive enchanting work. With it we would have the same problem the Asur do, not enough hands to make the things.