There are fewer organisms at the highest trophic levels, because

Because there is less energy at higher trophic levels

Explanation:

In a food chain that shows the feeding levels of organisms in an ecosystem, energy decreases as you move from the lowest trophic level to the highest trophic level.The lowest trophic level consists of producers which have the highest energy as they trap energy directly from the sunlight.The energy is lost from one trophic level to the next up to the highest trophic level which has the lowest energy.Therefore, organisms at the highest trophic level are fewer in number but large in size.