Question

Why do we know so little about the tribal societies and group of the medieval period

Answer 1

because they had their own customs and , rituals and beliefs
they did not follow the caste rules laid down by the Brahmans .
there was more equality among them.
they were not interfairing in matters of others.

Answer 2

We have little knowledge about the origin and subsequent history of the numerous tribes of India in the absence of sufficient archaeological data. The tribes preserved rich customs and oral traditions.These were passed from one generation to the next. Of late, historians have started using such oral traditions to write tribal histories.