It is because of inability of $n{s}^2$ electrons of the valence shell to participate in bonding that:

This phenomenon is known as the Inert Pair Effect. In the heavier elements of group 14 ($Sn$ and $Pb$), the $s$ -electrons are held more tightly and are less likely to participate in bonding.For $Pb$, the $+2$ oxidation state is more stable than $+4$. Thus, $P{b}^{4+}$ readily gains electrons to become $P{b}^{2+}$, making it a strong oxidising agent.For $Sn$, the $+4$ oxidation state is more stable than $+2$. Thus, $S{n}^{2+}$ readily loses electrons to become $S{n}^{4+}$, making it a reducing agent.