Why were northern districts transferred from Punjab to Himachal Pradesh in 1966?

In 1966, the Indian government reorganized the state of Punjab primarily to address long-standing demands for a state based on linguistic lines. Following intense agitation for a separate Punjabi-speaking state, the government divided the region into two new states: the primarily Punjabi-speaking state of Punjab and the mostly Hindi-speaking state of Haryana.1

The northern districts were transferred to Himachal Pradesh as part of this broader administrative restructuring. These areas, which included the regions surrounding Shimla, Kangra, and Kullu, as well as the district of Lahaul and Spiti and parts of the districts centered at Ambala, Hoshiarpur, and Gurdaspur, were identified as hill areas that were geographically and culturally distinct from the plains of the newly formed Punjab and Haryana states.21