Because the character behavior/personality can change up the plot or either a character could remove himself from the story/movie/novel
I have not read the story, but doing a quick search, I gather that the bat-poet changed as the story progressed. He began to see the world differently from the other bats, and tried to get the other bats to see things his way.
It showed bad character on Caesar's part, because it showed that he thought he was too good to take advice or insight from anyone. Even if he didn't believe what Artemidorus was saying, he should have at least taken the letter to read on his own time.
Is there any answer choice you can pick from?