Why did sudanic states develop in sahel and what advantages did they have

Because The Sahel was a fertile band, there was more agriculture, trade, and permanent civilization. The trade route established many colonies in order to bring gold out Africa and in to the Arabic world. 
Bantu as a common language, animism as a common religion, religion economics and history intertwined, and trade routes. 
Advantages: unity, trade partners.
Path from the Arab world to the western world of Africa for gold.
The further south you get, more agriculture develops.
Herds are going to be there in the south
Allows for more Sedentary life in the south.