its because the Food Chain follows the 10 Percent Law, where only 10 percent of energy is transferred from one trophic level to the other; rest is lost into the atmosphere.
Energy decreases as it moves up trophic levels because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms from the next level.
Explanation:
Energy is passed between organisms through the food chain. Food chains start with producers. They are eaten by primary consumers which are in turn eaten by secondary consumers. They are then eaten by tertiary consumers and in a long food day these can be eaten by quaternary consumers
