Why does creon decide not to bury polyneices?

He decides not to bury him because polyneices betrayed him, and his law is according to him higher than that of gods, which is why he will not be buried even though the divines want him to be.

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.