Most organisms in the top levels of an energy pyramid require more food than those on lower levels. On top of this, each level of the pyramid looses energy gained from food. This means that there needs to be more lower leveled organisms for the ones on top to eat in order to gain enough energy. Because of this, there's only enough food for a few of these types of organisms.
I hope this helped! There are probably a few more details you can add, but this is the overall reason :)
