Every business has a scope and jurisdiction it is projected to operate. Some of these casinos may want to have a presence in your country. Maybe they might expand later to Nigeria but they might now have such plans. Some casinos might not also have the financial capacity to cover your country, so they are restricted to a few countries.

Another reason may be regulation and gambling laws. In some countries, the process of registering a casino is too cumbersome. Operators might even have to give bribes to government officials to secure licenses. We also have some casinos that localize the games they have so they will have to cover areas where the games they offer are popular.