After independence, the Chief Commissioner's Province of H.P. came into being on 15 April 1948 as a result of integration of 28 petty princely states (including feudal princes and zaildars) in the promontories of the western Himalaya, known in full as the Simla Hills States and four Punjab southern hill states by issue of the Himachal Pradesh (Administration) Order, 1948 under Sections 3 and 4 of the Extra-Provincial Jurisdiction Act, 1947 (later renamed as the Foreign Jurisdiction Act, 1947 vide A.O. of 1950). The State of Bilaspur was merged in the Himachal Pradesh on 1 April 1954 by the Himachal Pradesh and Bilaspur (New State) Act, 1954. Himachal became a part C state on 26 January 1950 with the implementation of the Constitution of India and the Lt. Governor was appointed. Legislative Assembly was elected in 1952. Himachal Pradesh became a union territory on 1 November 1956. Following area of Punjab State namely Simla, Kangra, Kulu and Lahul and Spiti Districts, Nalagarh tehsil of Ambala District, Lohara, Amb and Una kanungo circles, some area of Santokhgarh kanungo circle and some other specified area of Una tehsil of Hoshiarpur District besides some parts of Dhar Kalan Kanungo circle of Pathankot tehsil of Gurdaspur District; were merged with Himachal Pradesh on 1 November 1966 on enactment of Punjab Reorganisation Act, 1966 by the Parliament. On 18 December 1970, the State of Himachal Pradesh Act was passed by Parliament and the new state came into being on 25 January 1971. Thus Himachal emerged as the 18th state of the Indian Union.
Why did the  Chief Commissioners Province of HP come into being?
as a result of integration of 28 petty princely states