User blog comment:Ixarising/Speculations, theories and predictions/@comment-37.15.180.124-20140807002430/@comment-1359322-20140807005136

It's easy, because they don't know what will happen to Issei's body if he were to take in either of those jewel. His life force is unstable.

Anyway regarding Grendel, it's because the jewel is no longer related to Sacred Gear. For the Divine Dividing jewel, it was still part of a Sacred Gear thus making it possible. But when it was used to seal Grendel, it no longer had any of the Boosted Gear or Divine Dividing nor is it a part of the Sacred Gear system anymore.

Also Evil Dragons are extremely persistent, there's a high possibility that Grendel might take over the Boosted Gear and Issei's body, so it's not a wise decision to do so.