Because the foreign/local online casino must obtain a license from the Nigeria gaming commission before they are able to operating in Nigeria.
and:
The legal games are lottery, land-based casino, and sports betting. Roulette, dice games, and non-skilled card games are illegal.
Source: https://en.wikipedia.org/wiki/Gambling_in_NigeriaPerhaps that is a reason why some of the casino providers dont want to apply/extend a license there, because live casino games, original games and maybe slots too are considered illegal there.
That's a very good answer that covers the entirety of the OP's question.
Casinos have to operate under guidelines of their licenses and nobody is going to look at statistics that say how many people in that country gamble. It doesn't matter to a clerk that issues the license. Most likely it's due to internal regulations of a country that other countries (license issuers) don't want to violate, or there's a problem with unstable currency/government and they just don't want to deal with potential loss by a player/investor as they'd have to handle complaints.