Why do hydroxyl compounds of heavier oxygen group elements become more stable down the group?

The stability of hydroxylated compounds of the oxygen group (chalcogens) increases as the size of the central atom increases down the group.1

For lighter elements like oxygen, the formation of hydroxylated compounds with positive oxidation states is resisted. In the case of sulfur, while lower-state derivatives exist, more highly hydroxylated compounds such as ${\text{S(OH)}}_4$ and ${\text{S(OH)}}_6$ do not exist. This is primarily due to the high charge density of the sulfur atom in these states—the large number of positive charges relative to the small diameter of the atom—which repels the electropositive hydrogen atoms. Additionally, the crowding that occurs when multiple oxygen atoms are covalently bonded to a small central atom favors the loss of water.1

As the size of the chalcogen atom increases down the group, these steric and electrostatic constraints are reduced, allowing for the existence of more highly hydroxylated species, such as orthotelluric acid, ${\text{Te(OH)}}_6$.1