From the official WoW blog:
Once we decided to no longer offer new head enchants, we made the older ones non-functional to eliminate giving players the feeling that they had to go back to older content or miss out on a little bit more power. Head gear is simply no longer enchantable, and you now have one less required step to get a piece of loot ready to wear.
Hurrah for reducing "forced" faction grinds. And double hurrah for ensuring that you don't have to go back a grind expired content just because your raid leader is a min-maxing dickâ¦ ;-)