Answer:
Because of timid and less aggressive nature of Johnston and aggressive nature of Hood.
Explanation:
The Atlanta Campaign was the military campaign fought between Union General William T. Sherman and Confederate general Joseph E. Johnston, in May 1864 in the North Georgia, during the U.S. Civil war (1861-65).
General Sherman attacked Georgia from the routes of Tennessee. The army of Tennessee was led by the army commander Joseph E. Johnston. Johnson was a timid and less aggressive army commander, and Sherman knew this weakness of Johnston, and knew that they had great chances of winning the battle.
In late July, Jefferson Davis, who was the president of Confederate, feared that under the command of Johnston, Atlanta would be defeated because of his less aggressive nature. So, Jefferson replaced Johnston with John B. Hood. Hood was an aggressive army commander, he promised to not let go of Atlanta and will defeat Sherman.
Because of this aggression of Hood, which is required in an army commander for this battle, Jefferson fired Johnston and replaced him with Hood.
