The basic reason that the United States did not comply with the 1954 Geneva Accords was that they did not believe that their side would win the elections.
According to the terms of the Accords, the country of Vietnam was supposed to be reunited after elections. Both the North and the South would vote to see who would govern the whole, reunited country. However, the United States was worried about the likely outcome of the vote. They knew that Ho Chi Minh, the leader of the North, was very popular. They also knew that Ngo Dinh Diem and Bao Dai, the Southern leaders, were not popular. Because of this, they realized that the election would lead to a "bad" result. This caused them to ignore the Accords and simply allow the North and South to remain separate.