It is because of inability of ns2 electrons of the valence shell to participate in bonding that (NEET 2017)

Inability of ns2 electrons of the valence shell to participate in bonding, on moving down the group in heavier p-block elements is called inert pair effect. Thus, Pb(II) is more stable that Pb(IV)and Sn(IV) is more stable than Sn(II). Therefore, Pb(IV) is easily reduced to Pb(II) and Sn(II) will be easily oxidised to Sn(IV). Thus, making Pb(IV) as oxidising agent and Sn(II) as reducing agent.