Why only one Ppi-Ppi bonding takes place in SO3?????????????????????

The hybridization is sp2 .The electronic configuration of sulphur is

GS:1s2 2s2 2p6 3s2 3p4.

ES:1s2 2s2 2p6 3s2 3p3 3d1

So by valence configuration sulphur has one lone pair in s orbital .It forms two sigma bonds with two oxygen atoms with two electrons in p orbital.

Remaining one electron in p orbital forms a pi bond with one p electron of one oxygen resulting in p-Pi p-Pi bond.the p orbital contains Of Px Py Pz.one of these orbitals forms pi bond with same p orbital of another atom