Why doed d-orbital show poor shielding effect?

Screening effect: The intervening electrons can screen the valence electrons from the nuclear attraction. Screening effect depends on the orbital in which intervening electrons are present. Normally, s-electrons have greater screening effect than p-electrons and p-electrons have greater screening effect than d and f-electrons.

As d-orbital has poor distribution of electron density around the nucleus, d-orbital show poor shielding effect.