Your question is based off a false premise. Neutron interactions have nothing to do with isotopic mass. The isotope with the highest neutron interactions cross section is Xe-135. Lead just happens to not be that good. Other elements than interact strongly with neutrons are beryllium, carbon, hafnium, cadmium. On the other hand deuterium and helium have fairly low interaction cross section. Water is good because it's very cheap and has lots of protons which are good at shielding neutrons. Same reason concrete is used. Cheap and abundant. Also doesn't because significantly radioactive when interacting which is another plus for shielding.