As far as I am aware this is because the Monocle is in fact a piece of armor (with stats – exotic from memory) not a skin (without stats, and directly applicable to items).
However comparing it to the Zephyr helms, I am far away from my computer at the moment and can’t confirm this, but I don’t think that they have stats and are an unlockable skin item only (at least post patch in the current content).
In saying that I kind of wish it did unlock all three armor types. But alas.
Edit: I believe it also may have something to do with what Khani said, as this is also relatable with the fused gauntlets. You pick one, but unlock all armor types, however this item is not tradable.