CO2 and SO2 both are triatomic molecules; but there is a big difference in their dipole moment. why?

S has d-orbitals while C does not have any and hence CO₂ forms multiple bonds and has a linear structure. SO₂ has a electronic tetrahedral geometry with two lone pair of electrons giving it a bent molecular geometry.  Dipole moment is a vector quantity and in the linear CO₂ molecule the bond moments are equal in magnitude and opposite in direction so they cancel out giving an overall dipole moment of zero.  In SO₂, the bond dipoles of each S-O bond do not cancel each other.