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.
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