A Biafran autonomous region implies a homogenous, united, Nigerian nation outside of the Biafra region. Biafra could gain power from the capital (Lagos back then) within a broader federalist framework for Nigeria, but it would be strange for only Biafra to gain autonomy.
Since independence, the number of states in Nigeria has increased from 3 to 36. The country is almost evenly split between Muslims in the North and Southern Christians, with each religious group comprised of several distinct religious groups (Igbo, Hausa-Fulani, etc.). Nigeria's internal diversity is like an African analogue of the British Raj gaining independence as one state.
Nowadays Nigerian democracy has stabilized and the Northerners and Southerners alternate control over the presidency, but Nigerian politics is still largely defined by North vs. South cultural divides.