General McClellan was reinstated as commander of the army of the Potomac due to his ability to be able to do what

Lincoln was desperate for an actionable general after John Pope loss at the second battle of bull run.

the general was quite decisive outside the battle ground and often failed to deliver convincing results in the battle field which often made Lincoln frustrated. Lincoln had called in Pope, who seemingly failed disastrously. Lincoln was in dire need of a decisive general which explains why he recalled General McClellan who surprisingly was able to defeat the confederate forces.