Colonialists were not fans of pro-self determinationists (Awolowo, Enahoro, Azikiwe & co) during the struggle for indepence. The British felt Nigeria and even their other colonies in Africa were not ready but with pressure from Liberals in their government they had to offload certain burdens whilst retaining the opportunity to exploit these colonies.

The best way to do this was the military. Dome times the easiest way to get what you want some times is to point a gun afterall no one wants to die. The military then had more conscription from Northern Officer as the British did not meddle in Islamic customs, the Northerners were easier to work with because they were lettered (literate) through Islamic Education which was in its own way sufficient enough for the populace.
Western Education outside of the military in the North was discouraged by the ruling oligarchs of that time.
Independence for Nigeria then from a British colonial officer's point of view would have been confusing because although the North enjoyed a strong relationship with the West through trade the south was at the frontlines of the battle. The North was 'handed the Master's whip' if you know what I mean, this is why even today the North has more military conscriptions in Nigeria.

The End.