Why are there fewer top carnivores than herbivores in most land ecosystems?

This is because of the energy transfer within the food chain

Explanation:

When energy is passed in food chain only a small amount of the energy becomes new body mass (or is used in metabolic processes) and the rest of the energy is lost as waste. So, this means that a carnivore has to eat many herbivores that are below them in food pyramid in order to get enough energy for survival and growth.