He died because his infection was ignored at first, which cost him days of care. He also did not receive ZMAPP, the drug used to cure Missionaries Brantly and Writebol of Ebola because the supply was exhausted. I don't know if he received a transfusion either. (A transfusion from a previously infected but cured patient would have contained anti bodies and help with the cure).
http://www.usatoday.com/story/news/n...ment/15443441/