@EgyptianAmerican
The real reason behind this coalition is to curb the Iranian influence. So, why are these African countries are in it?
First of all, East Africa hold the red sea, they're right next to Saudi Arabia and Yemen and hold many Islands in between them. This is important to Saudis and Emirates of course if they want Yemen totally surrounded and locked enabling them to ward Iran off the region, do good damage to Houthi and to ensure East African government won't have soft spot for them (hence the massive aids they give to them and military base they built recently).
For West Africa, it was a pretty big news that two years ago, Nigeria and the neighboring countries were suffering from Shiite militant groups, the massacres by the military and they already accused Iran of supporting these proxies who were trying to incite strife in their country. That was not my word but the government in those countries. It was no coincidence that some of the countries joined the Coalition because of that so they share common interest with the Saudi led Coalition.
The same for Sudan where relations between the countries had deteriorated sharply. In short, a lot of Muslim dominated African countries are affected by this.
So, Saudi Arabia view African countries more strategically important than the Balkan. Also, I believe Saudi Arabia have pretty good ties with African countries unlike the Balkan states. Balkan states are not affected by this, they don't have the problems that Middle East, West Asia and some of the African countries have and are totally disconnected from the MENA region so it would make no sense for them to join.