User blog comment:B214/Volume 23 discussion blog/@comment-6341273-20170324115421/@comment-31137559-20170324151619

I actually believe it's due to his lack of natural talent. The Boosted Gear is more of a support Sacred Gear. Issei had nothing to him except for the little demonic power he had at the beginning. His base stats were very low. For example, Issei couldn't even do a complete Juggernaut Drive.