Because poison can't pass through such iron. And to have some advantage over Volcanic Plate set on certain situations.
EDIT:
Title changed.
And, something more:
Metalic Plate Shield
Gray/Black instead of brown
Without the crown holograph.
Normal defense equal to Ironmight Plate Shield
Stun resistance equal to Ironmight Plate Shield
And poison resistance.
But Ironmight has piercing defense...