Why do people from Bihar and Jharkhand call themselves north Indians?

Bihar/Jharkhand are the only one states in Eastern part of India where Hindi is the main language. Other Eastern states like West Bengal & Odisha are not Hindi speaking states. Before the re-organization of railway Zones, North Bihar was part of North Eastern railway Zone (Gorakhpur Headquarter) and Southern Bihar & Northern Jharkhand was part of Eastern Railway (Kolkata Headquarter). Even the Jharkhand team is part of Eastern Zone Ranji team. Bihar/Jharkhand are part of Eastern Zonal Council.

But geography doesn’t always play a part in culture. Eastern UP is known as Purvanchal (Eastern Area). Bihar’s culture is similar to that of Eastern UP. People of Eastern UP and Bihar/Jharkhand are called Poorabiya (people from East). When you read the history of 1857, Poorabiya world have been used for soldiers from Bihar & Eastern UP. Hindi (Khari Boli) was introduced in Bihar in 1890 in education. As language of Bihar was similar to Hindi, Hindi became dominant language in Bihar. A few of land papers (more than a century old) of my family is written in Kaithi Script (Kaithi - Wikipedia).

When Bihar was part of Bengal, Bengali speaking people used to dominate Government jobs (mainly Railways) and professional services (doctors & lawyer). As educated class, Bengali speakers were very visible in each and every corner of Bihar. Even today you will find sizable native Bengali speaker in almost all cities of Bihar/Jharkhand. The state of Bihar was demanded for establishing a different identity from Bengal. When Bihar became a state in 1912, it was a wise decision for residents of Bihar to club Bihar with North Indian rather than East India. West Bengal and present Bangladesh (earlier Bengal) used to dominate East India. And due to cultural and language similarly, Bihar fitted well into concept of North India