Answer by taking a quote from the following article:

Muhammad Zia-ul-Haq was born in a Punjabi Arain family in Jalandhar, Punjab State of the British India, on 12 August 1924 as the second child of Muhammad Akbar, who worked as a staff clerk in the Army GHQ of India Command of British Armed Forces in Delhi and Shimla, prior to the independence of Pakistan from British colonial rule in 1947. Most accounts confirm that Zia-ul-Haq came from a religious family and religion played an important part in molding his personality. His father was known as ''Maulvi'' Akbar Ali due to his religious devotion. He completed his initial education in Simla and then attended St. Stephen's College of the University of Delhi for his BA degree in History, which he graduated with highest marks in the college in 1943.

After assuming power as Chief Martial Law Administrator, Zia shortly appeared on national television, PTV promising to hold new and neutral parliamentary elections within the next 90 days My sole aim is to organise free and fair elections which would be held in October this year. Soon after the polls, power will be transferred to the elected representatives of the people. I give a solemn assurance that I will not deviate from this schedule. He also stated that the Constitution of Pakistan had not been abrogated, but temporarily suspended. Zia did not trust the civilian institutions and legislators to ensure the country's integrity and sovereignty therefore, in October 1977, he announced the postponement of the electoral plan and decided to start an accountability process for the politicians. On television, Zia strongly defended his decision for postponing the elections and demanded that "scrutiny of political leaders who had engaged in malpractice in the past". Thus, the PNA adopted its policy of "retribution first, elections later". Zia's policy severely tainted his credibility as many saw the broken promise as malicious. Another motive was that Zia widely suspected that once out of power the size of the Pakistan Peoples Party rallies would swell and better performance in elections was possible. This led to request for postponement of elections by the right-wing Islamists as well as left-wing socialists, formerly allied with Bhutto, which displaced Bhutto in the first place. Zia dispatched an intelligence unit, known as ISI's Political Wing, sending Brigadier-General Taffazul Hussain Siddiqiui, to Bhutto's native Province, Sindh, to assess whether people would accept martial law. The Political Wing also contacted the several right-wing Islamists and conservatives, promising an election, with PNA power-sharing the government with Zia. Zia successfully divided and separated the secular forces from right-wing Islamists and conservatives, and later purged each member of the secular front. A Disqualification Tribunal was formed, and several individuals who had been members of parliament were charged with malpractice and disqualified from participating in politics at any level for the next seven years. A white paper document was issued, incriminating the deposed Bhutto government on several counts. It is reported by senior officers that when Zia met federal secretaries for the first time as leader of the country after martial law, he said that "He does not possess the charisma of Bhutto, personality of Ayub Khan or the legitimacy of Liaquat Ali Khan" thereby implying how can he be marketed.

Why were the elections postponed
Zia did not trust the civilian institutions and legislators to ensure the country's integrity and sovereignty therefore, in October 1977, he announced the postponement of the electoral plan