This is because in biology, energy is transferred from organism to organism and some energy is lost along the way. Each energy transfer results in a loss of energy, so that energy is always at a lower level and can never be returned to its original high energy level. On the other hand, chemical elements can form products, then that reaction can be reversed and break back down into reactants. Additionally, the law of conservation of matter says that matter can't be created or destroyed, only transferred from one form to another.
