That's not a surprising thing actually, because there are several things that underlie it, for example in slot games it will depend on the slot provider in a particular country, it is available but it may not be in your country and maybe for sports betting it is a casino licensing factor although I'm not really sure about this.
The opposite happens in my country, I don't know for sure how many millions of people use online casinos, but gambling in my country is illegal, but many online casinos do not include my country as a prohibited list, and the government only takes precautions by blocking them through internet provider services.
Sportsbetting may not be available in their country, just like what you mentioned here about slots. Slot providers have their jurisdiction where they can be played, hence, some providers are restricting countries. Because why would sportsbetting be not available if your government has nothing against with it? That's a very simple logic to think about.
With what you've specified, I think a lot of gambling companies would want to offer services from bettors in that region but their are many things that would be hindering them, despite the population and the amount spent on gambling, for instance they might not fall in line with the taxation policy of your government that alone hinders a lot of companies from doing business in many countries.
Secondly is the regulation or fines form your country doesn't align with that of the company then they won't want to invest or do business in such country. But I believe there are many betting companies operating in Nigeria and I'm aware that the competition is very though over there among different betting companies but one of the most outstanding betting company over there is sporty bet.
We don't know exactly the protocol of his country towards sportsbetting. However, there's a valid reason behind it for sure. Because if there's no one hindering it, why would they are not offering the services of sportsbetting? Just check your local protocols towards online sportsbetting and you may understand such action.