[...]
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.
Yes this is likely the reason why. A lot of gambling platforms today offer both online sports betting and casino games so it makes little sense to even bother going all the trouble of registering in a country where their services is limited.