I'm drawing primarily on Inventing Vietnam: The United States and State Building, 1954–1968, by James Carter for this answer, and all my quotes will be from this book as well.
The US Secretary of State under the Eisenhower administration at the time, John Foster Dulles, was extremely anti-communist and believed that southeast Asia would be important if the USA wished to maintain its presence in Asia moving forward, both as a way to supply Japan and as a way to stop a domino effect following the "loss" of China. During the conference, Dulles's main objective was to "establish a collective security mechanism for the future defense of Southeast Asia". Towards this end, he wished to use the conference as a way to retain influence in the area, but also wished not to commit the USA to any concrete agreements that would restrain its freedom to do what they really wanted to in the region, and not to do anything that would recognize the Vietnamese communists. In the agreement they eventually reached at this conference, the USA effectively surrendered all of northern Vietnam to the communists under Ho Chi Minh, so they failed on this last item, but Dulles still saw the result of the conference as a victory since it created a legal foundation for the USA to start building up an "umbrella of protection" in southeast Asia, including south Vietnam. Dulles gave his word that the USA wouldn't try to subvert the 1956 election mandated by the Final Declaration, and then immediately set about creating this security collective, with the hope that south Vietnam would be strong enough by 1956 to be able to side-step these elections.
Why did the USA not want to commit to an election that would re-unify north and south Vietnam? To be blunt, they knew the Communists would win, and this would not help the American position in Asia. Shortly after the Geneva conference, an American intelligence report said that "if the scheduled national elections are held ... the Viet Minh will almost certainly win". In the time between the 1954 Geneva conference and the scheduled elections in 1956, they tried to prop up a strong south Vietnamese government under Ngo Dinh Diem as much as they could. When the time for the 1956 elections came, they were not held, as Diem and the Americans "cited conditions preventing the possibility of free elections and simply scuttled the mandate", knowing that they would almost definitely lose to the Communists. With this, "the Geneva Accords for Indochina became what Secretary Dulles envisioned in 1954: a stop-gap measure to buy time to develop a US-friendly regime below the seventeenth parallel. Once sufficiently in place, the restrictions imposed at Geneva became at least a nuisance and at worse a serious threat to the entire project."
In short, the Americans at the Geneva conference had a different aim from the other participants. Their goal was to set up conditions that would give them time to build a foothold in the southeast Asian region, and so they did not want to commit to anything that would interfere with this aim, and holding elections that would see their supported regime defeated definitely would not help them.