Armor has an unfortunate tendency to interfere with magic. Much of this has to do with the magic-absorbing properties of metal, but even hardened leather or the densely-packed fibers that one might find in padded armor can act as a sink for mana when a mage tries to cast a spell, soaking up much of the power that they gather from their environment.
Suits of armor have a Mana Dampening rating. While wearing armor, your Mana Capacity, the maximum number of charges of Mana Flux that a character can hold, is reduced by the armor’s Mana Dampening rating.