Why was diem overthrown?

Diem was overthrown because he was an unjust ruler, who had ignored his people, and gained his power through terrible means. He was more interested in having power, and his own personal gain than he cared about the people. When the people began to rise up they were violently put down (e.g. the Buddhist protesters). In his rise to power, he had won a staged election in which he cheated his way into eliminating any competition, Dr. Dan being the favored leader by a landslide. After so much abuse, the even his own Generals had decided to rise up and they staged a coup, ending his tyrannical reign.