Because the author is wary of power creep. Enchantment with duster = modern body armor. Enchantment with modern body armor = ?
Shouldn't be a problem. Armour doesn't stack.
I misspoke... protection equivalent to the best body armor. It's better overall because it isn't obviously armor, so you can wear it in public without getting the police after you, and the White Council doesn't want to be publicly known. I think it's also significantly lighter, though I'm not 100% sure on that.
Meh. Wardens carry full-sized swords and throw fireballs, once they gear up for battle subtlety is a lost cause.
Unless, of course, they choose to turn invisible. But armour wouldn't affect that.
So the armour is about as effective as the duster, and it costs nothing where the duster costs a great deal of effort. (Okay, not actually nothing. But money shouldn't be a real issue for the Council.)
Harry's duster works well for him, since he's a detective and he has to fear assassination attempts, but I think I'd prefer something else for my soldiers.