It is because of the inability of ns ^ 2 electrons of the valence...

The inability of ${ns}^{2}$ electrons of the valence shell to participate in bonding on moving down the group in heavier $p -$block elements is called inert pair effect.

As a result,$Pb ( II )$is more stable than $Pb ( IV )$

$Sn ( IV )$ is more stable than $Sn ( II )$

$∴$ $Pb ( IV )$ is easily reduced to $Pb ( II )$.

$∴$ $Pb ( IV )$ is an oxidizing agent.

$Sn ( II )$ is easily oxidized to $Sn ( IV )$

$∴$ $Sn ( II )$is reducing agent.