Probably because of the way he thought he'd been unfairly passed over by Sherman in favor of West Point grad and Regular Army Maj. Gen. O. O. Howard for the vacancy at the head of the Army of Tennessee following the death of James B. MacPherson at the Battle of Atlanta. As senior corps commander, Logan took charge and successfully repelled the Confederate assaults; plus by then he had a long and honorable career with that army and wasn't an outsider like Howard, who it should be remembered had had a far-less-than-stellar career. Unfortunately for the service, postwar Logan was able to exact a revenge of sorts as head of (I believe) the Ways and Means Committee withholding funds for the Army - then headed by his old nemesis Sherman - during the Sioux War of 1876.