This is arguably more of a geography question than a history one, because its population density is not so very out of step with the rest of West Africa. Africa, remember, is an entire continent, so your question is a bit like asking why Japan is so much more populous than Mongolia, given that they are both in Asia. The simplest answer, therefore, is that it is in a densely populated region and is vastly larger than the other countries there. It also, as you might guess, largely runs within the basin of the Niger River, one of the world's great historical river systems.

I'm sorry I'm not giving a more complex or detailed answer, but I think when you see Nigeria in its proper context its high population is not so strange after all.

It is worth noting that, historically, Nigeria is where the oldest iron working and oldest statues in sub-Saharan Africa have been found (with the as yet little known Nok Culture, which made just the coolest statues EDIT: This is not a Nok statue, but a later imitation of the style).

The answer is basically water, and boundary divisions that leave Nigeria without a major zone of low population density.

Large river deltas can support extremely high population densities - by comparison, look at northern Egypt or Bangladesh. If you just looked at the green part of Egypt, its population density would be off the charts - but it's tempered by the huge percentage of the country that's practically uninhabitable desert. The delta where the Niger River drains into the Atlantic is very densely populated. When Biafra, located in the delta region, seceded from Nigeria in the 1960s, it comprised only about 8% of the country's area but more than a quarter of its population.

The other West African countries with similar geography to Nigeria (Ghana, Cote d'Ivoire, Togo, Benin) are also fairly dense but lack the huge, fertile delta that Nigeria has.

Nigeria is also unlike those countries in that it has extensive territory in the sahel zone (the ecological borderlands south of the Sahara). The sahel can be surprisingly population-dense - it's a sort of "Goldilocks zone" between the bone-dry Sahara and the humid forests to the south, although climate change is already starting to destroy this.

But finally, take a look at this map. You can see that Nigeria, while very dense, is not notably more dense than some other parts of the continent (Lake Victoria, Ethiopia, the Nile Delta, the entire Maghreb region stretching along the coast from Morocco to Libya). If you redrew the borders so that you only included the most populated areas inside a country, you could easily equal or surpass Nigeria in density. If Nigeria included the territory of Niger to its north (which it easily could have, if didn't happen to be colonized by separate European powers), it would have a much lower overall density.