Defective analysis. ZAB was leader of West Pakistan. Bengalis at that time were fed up with West Pakis atrociities (dicatorships) hence they freed from the bondage of dictators/West Pakis. ZAB was most popular figure in Punjab, Sindh and partly in NWFP. Baluchistan was not a big province to wiegh against Punjab and Sindh. But he committed blunder when he started military operation in Baluchistan to please Punjabis. He was very much popular as long Baluchistan was ruled by BNP (someone correct me on this) headed by Ghous Bux Bazenjo and NWFP was ruled by Mufti Mahmood of JUI. He destabilized these governments and tried to install its own PPP governments in both provinces and failed miserably.
However he again won elections of 1977 but mulla parties PNA alleged him of rigging and started movement with the help of army and ultimately his legal government was dismantelled and later on mardood Zia-ul-Haq hanged him created permanent cracks between Punjab and Sindh provinces.