It's because having Magic Power (the fuel for Magic) increases the resistance when absorbing Mana in the air, which is the fuel for Magecraft. As a Magic Powerless, Drossel has absolutely no mana inside her body, which makes her completely inept in Magic. In exchange however, she has complete access to the mana in the air, making her a sort of Mana Sponge. Fundamentally speaking, both Magic and Magecraft do the same thing, but they use incompatible resources.