I think I understand why the DLC armors are NOT upgradable ... and here's why: They are easily scavenged at the start of the game, and are really only there to give you some "buffs" while getting your game going outside of the Great Pleateau. They will serve you well until you find stuff like the Barbarian Armor, Stealth Armor, etc. After you get those, you can "ditch" the DLC armor and start using the in-game armor and upgrade it.
Now, that said, Majora's Mask and the Korok Mask have unique qualities that will help the entire game, and don't need upgrades. Those alone would have been worth it.
On a similar note ... and not necessarily specific to the DLC "armors" ... but I think it's just plain silly that something like the Phantom armor - which appears to be a kind of "knight's armor" has an attack buff instead of a defense buff. Wearing that kind of armor (in reality) would make your attack power probably fade a little since you're using a good bit of strength to overcome the weight and lack of mobility in such armor, but the trade-off would be that weapons wouldn't be able to hurt you as much.
I'm not really trying to nit-pick too much here, but it would have been nice if more logic went into the qualities of certain armor.
Another example is the Hero's Tunic. Seriously people, it's a SHIRT. How can a simple shirt be upgradable to a level of defense that metal armor is? That makes no sense, even in a video game world.