Why does electronegativity decreases down the group even though the effective nuclear charge remains same?

Electronegativity, in simplest terms, is the ability of an atom to pull shared electrons (of a covalent bond) towards it. The electronegativity of an atom is dependent on two factors: effective nuclear charge and distance between the nucleus and electrons. Effective nuclear charge is the nuclear pull experienced by outer electrons, adjusted for repulsion offered by core electrons (electrons in the inner layers). As we down a group, in the periodic table of elements, the effective nuclear charge is same. However, the atomic radius increases down a group, which means electrons are now farther away from the nucleus. The sum of these two effects means a decrease in nuclear pull on electrons as we move down a group in periodic table of elements. And hence a decrease in electronegativity.

Hope this helps.