Question

why are the Mudville fans excited when Casey steps up to bat in Casey At The Bat?

Answer 1

hey i did that last week ! XD anyway there excited because he is the best player on their team.

Answer 2

They are curious about whether he will strike out. 
They wonder why a better player is not batting. 
They are sure he will hit a home run and win the game. 
They are worried that he will fight with the umpire about his calls.