Ahh, the new guy
welcome.
The accepted reason why Espers can't use magic is because the AIM field that all Espers produce would
violently reject any magic. This is more obvious with the Imaginary Number District that Fuse=Kazakiri creates. In the fight between Vent and Touma, Vent mention that the AIM field produced by her interfers with the normal flow of magic, causing her own spells to react badly with her.