Why there are So many people from Rohtas-Kaimur Region in Varanasi?

See I am living in Varanasi from 2 Years. I have met many people from Rohtas-Kaimur here who are living for generations. There is also a colony of people who belong to Kaimur-Rohtas region. Even Manoj Tiwari is from Kaimur. What is the reason why so many people of this region are in varanasi?.