The S=O bond is a polar bond,this is because the O-atom is more electronegative atom than that of the S atom. Hence, O pulls the bonded electrons towards itself more as compared to the S atom pulls, as a result of which the electron density is more towards the O-atom and the O-atom gets the partial negative charge and S gets partial positive charge.

As the electron density at S in SO₃is decreased, it is in need of electrons to supress the partial positive charge and hence, SO₃ act as an electrophile.