There were several reasons why few people migrated to the Terai region of Nepal until 1950:
Lack of infrastructure: The Terai region was primarily a dense forest with few roads, bridges, and other infrastructure. This made it difficult for people to access and settle in the area.
Malaria: The Terai was also known for having a high prevalence of malaria, which deterred people from settling there.
Agricultural viability: The Terai soil was not as fertile as other regions in Nepal, which made it less attractive for agriculture-based settlement.
Cultural and ethnic factors: The Terai was also home to several indigenous communities who had a distinct culture and language. This, along with other ethnic and cultural differences, made it difficult for people from other regions to settle there.
Overall, these factors contributed to the slow rate of migration to the Terai region of Nepal until 1950.