The reason is the system of government our great country Nigeria practices. Although our government is called a federal government, the system of government is actually Unitary. I will explain.

When Major General Aguyi Ironsi came into power in 1966 after the first coup d'etat organized by Nzeogwu, Nigeria was practicing true federalism. It had four autonomous regions(Northern, Western, Eastern and Mid-West regions) that paid tax to the federal government.

After Ironsi came into power, he scrapped the four regions and changed the system of government to Unitary in order to consolidate power (this was one of the reasons for the counter coup).

In the Unitary system, the federal government controls all the resources of the state. That means any resource in any part of the country regardless of region or state is explored by the Federal Government and the revenue obtained is shared to the states and local governments.

The welfare of the people is determined by how well the state governments can utilize the funds to the betterment of the people. Unfortunately good governance has always been lacking in our government.

It's important to note that the oil producing states get the lion share in federal allocations but there's very little difference in the standard of living in any of the states in Nigeria because of corruption and mismanagement.

The situation will be much different if we practiced true federalism. States will be more diligent in managing their resources since they only remitt a portion of the funds to the federal government.

This is the situation and is the reason the people in oil producing states are as poor as any other state.