Here's why:

* Oxidation State: SO₃ has sulfur in a higher oxidation state (+6) compared to SO₂ (+4). Higher oxidation states generally lead to stronger acids.

* Polarity and Bond Strength: The sulfur-oxygen bonds in SO₃ are more polar and stronger than those in SO₂. This increased polarity makes it easier for SO₃ to donate a proton (H+) and act as an acid.

* Stability of the Conjugate Base: The conjugate base of SO₃ (SO₃²⁻) is more stable than the conjugate base of SO₂ (SO₃²⁻). This stability is due to the delocalization of negative charge over multiple oxygen atoms in SO₃²⁻.

In summary: The higher oxidation state of sulfur, increased bond polarity, and greater stability of the conjugate base in SO₃ all contribute to its higher acidity compared to SO₂.