Here's why:

* Mercury's Common Oxidation States: Mercury (Hg) primarily exists in two oxidation states: +1 (mercurous) and +2 (mercuric).

* Mercury(III) is Unstable: A +3 oxidation state for mercury is highly unstable and doesn't readily form stable compounds.

Therefore, the correct formulas for mercury oxides are:

* Mercury(I) oxide (Mercurous oxide): Hg₂O

* Mercury(II) oxide (Mercuric oxide): HgO