Final answer:
Plato and Aristotle's philosophies waned in popularity due to their abstract nature and the shift towards empirical evidence and practical applications in philosophy.
Step-by-step explanation:
The philosophies of Plato and Aristotle did not remain as popular over time for several reasons. One of these reasons is that their theories were very abstract in nature, and as the world moved forward, people began to prioritize more pressing and tangible worldly concerns over philosophical abstractions. Additionally, these philosophies were not subjected to the scientific method of experimentation and observation, which was a significant drawback because it meant their ideas were not justified through empirical evidence. Furthermore, Aristotle's focus on materialism and the potential to improve politics through human understanding and invention signaled a move away from Platonic idealism towards more practical approaches to understanding the world.