This property is due to an effect called the inner pair effect.

In groups 13 and 14 as we move down the group the tendency of the s-orbital electrons to participate in the bond formation decreases this is due to the poor shielding of s-orbital electrons by the d and f orbitals. This is called an inner pair effect.

Thus,the +1 oxidation state in group 13 and +2 oxidation state in group 14 becomes more and more stable with increasing atomic number.