User blog comment:Miskos3/Fairy Tail Episode 241: Tartaros arc: Devil Reincarnation, Review/@comment-1359322-20150711141943/@comment-4933386-20150711150106

For Plot purpose? haha nah, the bomb will most likely pierce through the card and still cause a massive amount of damage. I doubt it has a stronger defense than Erza's Adamantine armor's shield.

Besides, I don't think Cana was trying to stop the bomb, she was trying to avoid the bomb, hence she put everyone in card and made Happy and co to fly them all to Cube as everyone is not as fast, the bomb is instant, and there are injured people.