Frost diagram and acidic/basic conditions.
Write out the half-reactions and you will see that oxidation of S almost always creates lots of protons.  So you can think of Le Chatelier's principle: if there are already lots of protons around, it will be harder to create more of them.  Here's the example for complete oxidation of sulfide to sulfate at pH ~7:

$$\ce{HS- + 4 H2O -&gt; 8e- + 9H+ + SO4^{-2} }$$

Conversely, under alkaline conditions, the equilibrium favors the products even more, because of the "removal" of the protons by the alkaline conditions.