A lack of incentives not to use the bomb - Weapons were created to be used. By 1945, the bombing of civilians was already an established practice. In fact, the earlier U.S. firebombing campaign of Japan, which began in 1944, killed an estimated 315,922  Japanese, a greater number than the estimated deaths attributed to the atomic bombing of Hiroshima and Nagasaki. The firebombing of Tokyo alone resulted in roughly 100,000  Japanese killed.
Responding to Pearl Harbor - When a general raised objections to the use of the bombs, Truman responded by noting the atrocities of Pearl Harbor and said that “When you have to deal with a beast you have to treat him as a beast.”