Probably for the same reason that AgCl is insoluble compared to NaCl and KCl. Trying to research this on the web gives all sorts of explanations about differences in lattice and solvation energies with no reasons why. My explanation is that the relatively exposed valence orbital because of the filled d orbitals inefficient shielding, the ionic bond has some charge transfer feedback increasing the lattice energy. This loosely explains the decreasing solubility and increasing color of AgBr and AgI and the relative solubilities of the PbCl2, PbBr2 and PbI2.