The best explanation for why there are fewer secondary consumers than producers is:
Around 10% of the energy is transferred to support the next trophic level.
This statement reflects the concept of energy transfer efficiency in an ecological pyramid, where typically, only about 10% of the energy from one trophic level is passed on to the next level. As a result, there are fewer secondary consumers than producers because energy diminishes at each successive level, leading to fewer organisms being able to be supported as you move up the trophic levels.