The main reason we do not have more information about the Indus Valley civilization is that archaeologists and historians have been unable to decipher any of the Indus Valley writing. This has made it challenging to understand their culture, administration, and daily life, as written records are crucial for reconstructing historical narratives.
While it's true there are fewer excavated sites compared to those in Mesopotamia and Egypt, there are in fact several major cities of the Indus Valley Civilization, such as Mohenjo-Daro and Harappa, that have been discovered and explored. However, the inability to read their script significantly limits our knowledge.
The notions that ancient peoples of the Indus Valley purposely obscured evidence or that scholars prefer other civilizations are less substantiated and do not fully address the key issue of the unreadable script.