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

This is due to the Inert Pair Effect, where the $s$ -electrons become increasingly stable and reluctant to bond as we move down Group 14.For $\text{Pb}$ ($Z=82$), the $+2$ state is more stable than $+4$. Thus, ${\text{Pb}}^{4+}$ acts as an oxidising agent to become stable ${\text{Pb}}^{2+}$.For $\text{Sn}$ ($Z=50$), the $+4$ state is more stable than $+2$. Thus, ${\text{Sn}}^{2+}$ acts as a reducing agent to become stable ${\text{Sn}}^{4+}$.