The reason for this is simple. No one knew what an atomic bomb was at that time.
The Manhattan Project that was making the atomic bomb was one of the most closely guarded projects of the war. The US certainly did not want the Axis powers to find out about it for fear they might get to work on their own atomic bombs. This fact, and the fact that no atomic bomb had ever been created, meant that very few Americans knew that it was even possible to create an atomic bomb. Since this was a completely new technology that was more powerful than any then known, Truman needed to explain what it was and how powerful it was.