Why is the biomass of herbivores greater than the biomass of carnivores?

In order for a food chain to be stable, there must always be less biomass as you go up in trophic levels (i.e. from plants to herbivores to carnivores). This is basically because energy is always lost as it is transferred to each successive level, since herbivores use some of the energy they get from plants to stay alive (leaving less for carnivores who eat them), and so there will always be fewer carnivores than herbivores.