Vibranium fell from the sky, landed in Wakanda and mutated the flora and fauna. T'Challa drank the heart shaped herb juice at least 3 times that we know of. Shuri made him that Vibranium suit that absorbed kinetic energy and released it back as a Vibranium shockwave. His body was exposed to alot of Vibranium radiation/energy. They never specified what disease he died from. It was probably Vibranium poisoning.