Because in middle ages after the fall of Western Roman Empire years of turbulence led to lost of knowledge, which was not in a focus of barbaric rulers until the period of Charlemagne the Great. That is why in those dark ages we have lost many things that ancient scholars left us.
Explanation:
After Roman Empire collapsed in 476, many Barbaric kingdoms were created on its soil. Most of them didn't lasted that much, such as Kingdom of Vandals or Kingdom of Ostrogoths. Most of them loathed and destroyed the heritage of Roman Empire and that is why for centuries we have lost many important information on that period.
