It may sound far too obvious, but the most likely reason is that George McClellan was hesitant to attack because he feared failing.

You see, McClellan lived a rather charmed life. He was born to a wealthy family, attended private schools, and was even granted special permission to enter West Point two years early. He gained renown for his engineering achievements in the Mexican-American War, but he never directed battles or got into the hard stuff of war. He was actually sick during most of the battles. He was an academic who translated French military manuals and seemed to have a gift in dealing with people. In 1857, he was sent to observe the Crimean War. Afterwards many in the nation considered him one foremost experts on modern warfare despite only being in his early 30s. By the time he arrived in Washington in 1861, he was already considered the city's savior for his construction of a massive series of forts around the city. After winning two small victories the press was calling him the American Napoleon. Yet even then his hesitancy showed. At both the Battle of Philippi and Rich Mountain he was extremely reluctant to commit to battle without overwhelming odds and would not risk reserves.

As to why, there are probably two major factors at play. The first seems to be a genuine desire to protect the men under his command and civilians near his army from unnecessary harm. This may have extended to some of the Confederate officers he faced across the field whom he still considered his friends. The other was a growing need for approval and affection from his men and the public. Within days of his early successes he was already writing to his wife about the many men suggesting he run for President or assume control as a dictator, or about how the President naturally deferred to his decisions. These are likely inventions of McClellan's or exaggerations of conversations which did take place. The praise, real or imagined, seems to have quickly gone to his head and clouded his decision making capabilities. This is not to suggest McClellan's praise was totally undeserved. McClellan was undoubtedly a bright mind who created battle plans that would have been successful had he ever bothered to implement them. He was also very popular with his soldiers and was instrumental in restoring morale after the crushing losses of First and Second Bull Run.

For a bit of fun, I highly recommend you listen to the satirical Letters Home from General George McClellan: A Dramatic Reading

However, it seems that McClellan was so worried about preserving his image and the lives of his men that he only felt comfortable engaging in battle when he felt he was guaranteed success. Even when he outnumbered the enemy nearly three to one he somehow convinced himself that the odds were even at best despite all intelligence telling him the opposite. McClellan would be given accurate troop numbers and positions but refuse to believe them. When public sentiment and political figures turned against him for his hesitancy he began to talk negatively about them publicly and refuse to cooperate for fear of sabotage or conspiracy.

In Battle Cry of Freedom James McPherson said that "Military success could be achieved only by taking risks; McClellan seemed to shrink from the prospect. He lacked the mental and moral courage required of great generals - the will to act, to confront the terrible moment of truth on the battlefield. Having experienced nothing but success in his career, he was afraid to risk failure." Men like Grant had faced great adversity, survived and grown from it. McClelland had known only success and was stunted by it. McClellan was likely simply afraid.