User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-77.194.131.32-20140725125535/@comment-25244540-20140725133853

I think its the fact the water clumped his sand together. Recall in One Piece how Luffy had to fight Crocodile's Sand Sand Fruit powers using water.

With the sand wet it was all clumped together, and therefore Keith had no way to spread his body because it got stuck together, essentially Juvia was the worst opponent he could have fought against.