The reason you can't is the original AF (artifact armour) set gained at 45-50 is unique and quested, whereas the later AF armour (level 50 AF1.5, level 60 AF2, level 70 AF3 and now level 80 AF4) can be regained by simply getting the required tome currency or equivelant.

So therefore it's simply pragmatism at work here: the original set is meant to be 'unique' and kept, whereas the higher level sets are basically little more than vendor trash in SE's eyes, thus you're meant to simply dispose of them if you have no more room by trading them in for Company Seals at your Grand Company and then just buy them again with the relevant currency. Either way, I don't see SE changing this.