The stability of +6 oxidation state decreases down the group in Group 16 is due to:
Resonance
Inert pair effect
Mesomeric effect
Inductive effect
Due to inert pair effect, the stability of oxidation state decreases down the group as the s-electrons are unavailable for the formation of bonds because the s-orbitals held close to the nucleus and the electrons held strongly by nucleus due to maximum electrostatic force of attraction.