Complex sentence: His existence is disputed and rejected by Shiite historianss because of the contemporary political conflicts between the divinely appointed Imamah of his father and brother and the feudal leadership of the Abbasids.
Split into simpler sentences: His existence is disputed and rejected by Shiite historians. The fact why his existence was hidden, was because of the contemporary political conflicts between the divinely appointed Imamah of his father and brother and the feudal leadership of the Abbasids.