Inconsistency is one big factor. I think it is a reflection of who we are as a people and our socio-political environment.
Another is the fact that I do not think Nigerians even view the possibility of being marketable. For an athlete to be "endorsable", they need to have certain qualities that align with certain products...but without consistency, it doesn't even matter is an athlete has all the other required characteristics.

gotta be known for something at least.