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 concept is known as the "10% rule" in ecology, which states that when energy is transferred from one trophic level to the next, only about 10% of the energy is passed on, while the rest is lost through metabolic processes, heat, and other factors. Consequently, this results in fewer organisms at higher trophic levels, such as secondary consumers, compared to producers.