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You made mention that the decision to give priority to northern projects is irrespective of WHO is in power, who is handling the contract or who is in charge of supervision of the said project. This tells you it's not intentional motive by leaders from the north to undermine the development of the south.
I believe it mostly has to do with the pressure the north puts on the authorities concerned once a project is awarded. You can imagine the state of federal roads in SE compared to generality of roads in the north. The elite did nothing to get the roads fixed in fact many of them participated in looting funds for the projects.
Also the relatively more conducive environment compared to the south. It's common for community people to tax constructors throughout the duration of project construction in the south, something you'd never see in the north.

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