User blog comment:B214/Question & Answers 5/@comment-31.57.87.86-20150601070101/@comment-24098614-20150602060020

The reason for Issei not using the Juggernaut Drive is simple the original chant, and the purpose of this power is more of a curse than a blessing because it draws upon the strength of the Demonic power of the person or in Issei's case his lifeforce because Vali can use without a problem but with a Time limit but Issei doesn't have this luxury since his Demonic power is simply pathetic compared to Vali's huge reserves.

The curse is mainly centered around the past users of the Sacred Gear remember the lines that Issei spoke using words like Domination because the power carries the hatred of the past users, and when Issei used it it basically depleted most of his lifeforce, which was quite substantial because it left him with a 100 years of Human life.

This pivotal event changed everything for Issei because even now he has to take Senjutsu treatments because of how unstable his lifeforce was after his new body was given to him. The event also led for Issei to drastically change the way he uses the Juggernaut Drive because the original way was deadly to him so he created a Juggernaut Drive that isn't harmful to him by radically changing the way it was used before.