According to one source, the major elemental constituents of cremation remains are calcium and phosphorus, presumably from the hydroxylapatite in bone (which consists primarily of calcium phosphate). The rest is mainly assorted trace metals, which exist in relatively minute concentrations in the human body and don't vaporize at the temperatures typical of a crematorium furnace.
Virtually all organic matter, comprising mainly oxygen, carbon, hydrogen, nitrogen, and sulfur, is oxidized into a variety of simple compounds and is removed as a mixture of gases in the furnace fumes (and perhaps some finely dispersed, light solid particulates).
Consequently, what remains is largely inorganic and non-calorific in nature. While the mineral and metal content of the ash can be dissolved and subsequently absorbed by various organisms in trace amounts, there's probably little or no organic material left that can be metabolically decomposed by bacteria, fungi, or plants as a direct energy source. Of course, there are chemical processes that can further break down the ash and isolate specific components via redox reactions (dissolution in strong acids to give salts, electrolysis, etc.), but obviously that's distinct from biodegradation.