Here's why:

* Sulfate is the polyatomic ion formed from sulfur and oxygen: SO₄²⁻

* The sulfur atom has a +6 oxidation state, while each oxygen atom has a -2 oxidation state.

* Since there are four oxygen atoms, the total negative charge from oxygen is -8.

* To balance this out and achieve a neutral overall charge, the sulfur atom needs to have a +6 charge.

* This results in a net charge of -2 for the sulfate ion.