It is because of inability of ns2 electrons of the valence shell to participate in bonding that

The inertness of s -subshell electrons towards bond formation is known as inert pair effect. This effect increases down the group thus, for Sn, +4 oxidation state is more stable, whereas, for Pb,+2 oxidation state is more stable, i.e., Sn2+is reducing while Pb4+ is oxidising.