Why can speakers of different tribal languages in Arunachal Pradesh have difficulty understanding one another?

Speakers of different tribal languages in Arunachal Pradesh often find their languages mutually unintelligible because the state is home to approximately 50 distinct languages and dialects. Most of these belong to the Tibeto-Burman branch of the Sino-Tibetan language family, and the high degree of linguistic diversity among the state's numerous ethnic groups contributes to this lack of mutual understanding.1

Due to this linguistic fragmentation, residents frequently rely on Assamese, Hindi, and English to serve as lingua francas for communication across different tribal communities.1