Here's why:

* Br₂ represents a diatomic molecule of bromine.

* Diatomic molecules consist of two atoms of the same element bonded together.

* Oxidation numbers represent the hypothetical charge an atom would have if all its bonds were 100% ionic.

* Since both bromine atoms in Br₂ are the same element and share the electrons equally, there is no difference in electronegativity, resulting in an oxidation number of 0 for each bromine atom.