I think the reasons are fairly obvious. People remember great centralized empires. They remember societies with extensive written records. Cultures that impres

I think the reasons are fairly obvious. People remember great centralized empires. They remember societies with extensive written records. Cultures that impressed their conquerors. Areas with contiguity of centralized civilization. Monuments that are noteworthy on the global scale. When those things are missing, past societies fade into obscurity. Even in Europe, ancient cultures are mostly remembered from their interactions with Mediterranean and Middle-Eastern societies. European history effectively begins with the spread of Greco-Roman influence, which in many cases was well into the middle ages. Whatever there was before is now mostly forgotten.