He decides not to bury Polynices because he was a traitor.
Polynices and Eteocles were two brothers who fought for the throne. They both wanted to rule over Thebes (after their father, Oedipus, left), however, due to the curse, their rule wasn't peaceful. So they ended up killing each other. Their sister Antigone wanted Polynices to get a proper burial like Eteocles did, however, Creon was against it because he thought only Polynices was a traitor.
