For the obvious reason that he was highly professional, pushed power down to his subordinates and expected them to use it, and protected them from Washington and the politicians. McClellan never threw anyone under the bus, unlike Pope, Burnside, Hooker, Meade or Grant, but assumed responsibility. Even when he knew his subordinates had performed poorly, and was privately scathing of them (Sumner after Williamsburg, Franklin and Smith after the Seven Days, Burnside after Antietam) he never made a big deal of it.
Egotism simply means taking pride in ones accomplishments. McClellan's accomplishments were actually pretty great. It does not imply self-centredness, and indeed, McClellan was very selfless and effusive with praise for the accomplishments of his subordinates. To McClellan, he wanted competence from his subordinates, and nothing more.