User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20170204105622/@comment-1359322-20170204135348

Because Ise's soul is still that of a human. Regardless of the body, Ise's soul is that of a human. And there's a limit to how much his soul can withstand. Even Sun Wukong said it the soul is a fragile thing.

Also Ishibumi didn't do that because he wanted Ise to be weaker than Vali. He did that because he needed to show that Ise's constant power-up comes with an equal risk. Ishibumi has been giving Ise power-ups after power-ups to the point where Ise went from a normal human with no special powers to a person capable of fighting ultimate-class beings in half a year, 6 months. Vali and many the others had years of training prior. The balance isn't there.

So having too many power-ups without any side effects at all for Ise wouldn't be good for the story plus it would be borderline bullshit. Even Vali's DxD L came at a risk of shortening Vali's life, so if there's anything Ishibumi is trying say, it's that excessive power-ups is dangerous.