As for the reason it's because the nature of Esper in this setting. Esper in here refer to "people who were born with great affinity and aptitude to [Pure Energy] and possess it inside their body" thus allows them to bend those energy into the shape they're good with. Meanwhile, Mages execute magic through their Madou that utilize [Pure Energy] in the air.

Esper cannot use Madou to cast magic because their body also release their own kind [Pure Energy] that might interfere the mechanism within Madou.