Answer: because he is not wearing shoes
Explanation: In "American Born Chinese" by Gene Luen Yang, the Monkey King is not allowed to attend the party with the other deities because he is not wearing shoes. The Monkey King wants to attend the with gods and goddesses who rule over him. He thinks that he is equal to the gods and so wants to be treated as equal by them. He is extremely angry by this and makes his subjects wear shoes as gods do.