KOH is the base thus, it gives OH^– ions thus it cannot remove OH^– ions from reaction mixture but it adds on the concentration of OH^– ions.

So, an acid must be added but if a strong acid is added to the reaction mixture then in acidic condition the MnO₄ ^– formed reduces to give Mn²⁺ thus, HCl which is a strong acid and SO₂ which on treating with water forms a strong H₂SO₄ cannot be used for this purpose.

Thus, CO₂ which forms H₂CO₃ a weak acid reacts to remove OH^– but not that much acidic that MnO₄ ^– undergo reduction. Thus, CO₂ is used for this reaction for completion.