User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-78.48.214.26-20180524172921/@comment-28884330-20180524174534

Because even if he masters it the power increase will be too small to be relevant now. With how much Issei already used Triaina and CxC, it is probably already close to mastering and fulling mastering it will only be a negligible upgrade.