Well its like how it was explained back in the itachi vs team kakashi fight...
Genjutsu works by disturbing the flow of chakra within the target. As that granny said, it takes two people to fight a genjutsu user, because the second person can stop the disturbance by injecting their own chakra.

Sasuke targeted the jinchurriki, and the bijuu was able to use its chakra to stop the disturbance. I don't think this has anything to do with the bijuu being the 8-tails instead of kyubi.