According to the law of conservation of matter, matter is never created or destroyed. yet in a food pyramid the amount of biomass decreases as we move up through the pyramid, from producers to top level consumers. why?

For an ecosystem to be able to sustain itself, more energy/biomass is required by the lower trophic levels. The lower you go in the pyramid, the higher the energy requirement is because the lowest trophic levels are the producers and they are the ones supporting the ecosystem. 

Consumers simply don't require as much energy as the producers do because they aren't the ones responsible for maintaining a stable ecosystem, and they aren't expending energy to transfer energy to a higher trophic level in the food pyramid.