Good question.
It may have been his natural caution, and reluctance to move until everything was ready.
But there seems little doubt that Lee was in a vulnerable position, and could have been forced to surrender after Antietam/Sharpsburg in September 1862.
Many in Washington had come to mistrust McClellan's will to fight, and some suspected that he was a secret Confederate.
cause he can take advantage of LEEs army and take him as condfederacy prison
cause he can take advantage of LEEs army and take him as condfederacy prison
cause he can take advantage of LEEs army and take him as condfederacy prison
cause he can take advantage of LEEs army and take him as condfederacy prison
cause he can take advantage of LEEs army and take him as condfederacy prison
cause he can take advantage of LEEs army and take him as condfederacy prison
If you mean at Antietam, it was because he simply moved too slowly - as he always did in the opnion of Lincoln's cabinet.
In the General Election of 1864, he ran as a Democrat against Lincoln. (He had a good chance of winning too.)
George B. McClellan wanted to return to the US Army with a high rank. When the US Civil War began he had his chance. He was sponsored by the Republican Governor of Ohio, Dennison and had the support of General Winfield Scott. McClellan was commissioned as a major general of volunteers and was assigned to command regiments being formed in Indiana and Ohio.
General George B. McClellan was not happy with President Lincoln's initial Emancipation Proclamation of September 22, 1862. He learned of it on September 24, 1862. McClellan discussed possible responses to the proclamation with his closest confidants. As this was happening, officers loyal to McClellan, were said to be speaking about a possible military dictatorship. McClellan, however, was never involved in that. What he did do was to speak to his Democratic friends in New York City to determine how New Yorkers would respond if he openly opposed Lincoln's proclamation.
George McClellan, he was later fired for slow reaction time, hired back by Lincoln, then fired again for not doing anything when he had a chance to crush the Confederacy. And after he was fired I believe it was George Meade.
On October 13, 1862, President Lincoln sent General McClellan a long letter explaining the plans Lincoln believed were needed to defeat the South. Lincoln was almost sure that an aggressive Union response from McClellan's army would not be forthcoming. With that said, McClellan had every chance to keep his position if he even tried to execute the plans Lincoln sent to him.