why in p-block elements the lower oxidation states are favoured by heavier metals? also explain inert pair effect

The inert pair effect, is the tendency of the outermost s electrons to remain unionized or unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are 2 less than the group valency for the heavier elements of groups 13, 14, 15 and 16.

When we go down in p-block elements we can see the succesive elements have an increased d or f- shell. This d or f shell cause the inert pair effect. Actually the mentioned shells have a weak sheilding effect of nuclear charge so the nuclear charge easily attracts the electrons in the outer shells than the d or f shells. This makes the electrons in the outer shells become unable to participate the reactions taking place. This makes them inert hence the name inert pair effect.

The tendency of ns² electron pair to participate in bond formation decreases with the increase in atomic size. Within a group the higher oxidation state becomes less stable with respect to the lower oxidation state as the atomic number increases. 

We can see this in case of Pb. Pb has f shell which has a very low shielding capacity and it has 4 electrons in outer most shell but due to this effect Pb is more stable showing +2 valency state.