Why was General McClellan so slow to act in the U.S. Civil War and why wasn't he replaced sooner?

So we can take this as two parts. 1. is the factors that contributed towards McClellan's approach to generalship. 2. What other alternatives existed and hod did Lincoln and others act on them?

For 1. its helpful to look at a series of assumptions Mac labored under.

He was outnumbered by his enemy, usually significantly, confirmed in his mind by reports for the Pinkerton run intel network he established early in his command in the winter of 1861-62. And a consensus either shared or not forcefully countered by his senior subordinates who were either hand picked by him and personally loyal(like Fitz John Porter), or frozen out of decision making wherever possible if they were not(like Sumner or Heintzelman).
That the failure to reinforce his army more was a combination in poor leadership in the Executive, combined with intentional sabotage of the majority old Whig and Democrat led army by the Republicans in power. Thus able to purge the for failure in the field.
Failure and destruction of the Army of the Potomac in the field would mean either a total collapse of the war effort, or unleashing the Radical Republicans to make the war a matter of abolition and remove any possibility of a settlement and negotiated reunion.
Conversely abandoning the accepted standards of conduct and flying the black flag to practice total war by his army could accomplish the very same, especially as the theater would be the approaches to the enemy capitol of Richmond. Combined with his background in engineering this only made things more plodding, needing complex plans for works and positions and only launched when he felt he was totally ready. In stark contrast to his episodes of almost manic energy of reorganizing the army to take the field(such as in the days after 2nd Manassas).
He also seems in the end to just not have the stomach and temperament to command men in battle with all the horrifying reality and finality that involved. He commanded only a single real battle in person, Antietam, and that even at a great distance from real fighting, was absent the field most of the 7 Days, and was even safe on a navy gunboat in the lead up to Malvern Hill. That the AotP extracted itself on the Peninsula says much more about the quality of many of the Corps and Division leaders as opposed to the general commanding.

So on to the 2nd part. Remember his tenure in command lasted only 15 months, from late July 1861 to early November 1862. And in that time he also gained and then lost the title of General-in-Chief.

In the Fall of 1861 none doubted his efforts at crafting army out of the new forces arriving and the scratch force that had been assembled under McDowell for 1st Manassas. And when Winfield Scott half retired/half was forced out on November 1st 1861 McClellan got his job on top of the field command. And after a winter of prodding, and spring of more prodding by Lincoln and several debated and set aside plans Mac and his new army finally embarked on the Peninsula campaign. Which of course ended in defeat at the 7 Days in late June into July 1862.

McClelland had been a former regular, and railroad man. And had commanded forces in West Virginia in 1861 that in terrible terrain out maneuvered and defeated dispersed Confederate forces, commanded overall by Lee. But even then displayed a reluctance to commit to bold attacks even then, and was aided as much by better logistics and the unpredictable nature of the early war fighting forces as anything. These raised his profile at a key time, and Winfield Scott was a fan of his so he was called up to the big leagues as it were.

Following defeata and in the face of an escalating national war effort, and frustration with Mac, Henry Halleck is brought in to serve as G-in-C as a DC bound coordinator and administrator. He had been the senior leader out West and seen some success. All who knew him from the old army acknowledged his smarts, and office capabilities, but his decision making and firmness in those decisions would leave something to be desired. However McClellan had lost effective control of the national strategy weeks before when he left DC, meaning Lincoln and the cabinet were basically learning on the fly without a senior officer as go between. Difficult enough just in Virginia where 6 different independent field commands were present in whole or part!

Even then though while entrenched on the Peninsula still all was not lost for McClellan. But his continued refusal to move out, unless given 50k or more reinforcements as he told Lincoln and Halleck when they visited was problematic. He had already during the course of 1862 been provided notable additional forces, including Ambrose Burnside's corps sized command that had been securing coastal North Carolina. While other forces like McDowell's Corps in Northern Virginia were still attached to him but tied down as part of the multiple small commands engaged in fighting in the Valley and securing the Capitol. A source of friction between general and president.

Thus with an immobile general with the main field army, the AotP was ordered to evacuate the Peninsula and return to the Capital, over the objections to the risk of attack during the movement by some leadership, and the lost opportunity to renew the attack over the same or other approaches by others. And several Corps (eventually parts of 4) were taken from the AotP and attached to the new Army of Virginia. This was a consolidation of the various forces and departments in the Shenandoah Valley and nearby and given to John Pope, who came from out West after service under Halleck and some minor victories. Lincoln had also at this time offered command to Burnside, he was an attractive choice as a successful independent leader, and known to the AotP leadership but not of them, and a personal friend of Mac. He declined as not being up to the job and out of loyalty.

Pope's forces included many units plagued by low morale, having been whipped by Jackson that Spring, and were essentially the black sheep of the Eastern theater. Pope was also known in the old army as a braggart and lived up to it, with notable orders essentially giving free reign to his ramshackle forces to live of the land of the enemy. Which conversely made it harder for some of his subordinates to shape their commands into effective disciplined fighting troops.

He also took a swipe at McClellan in a famous message to his command in mid July 1862.

Meantime I desire you to dismiss from your minds certain phrases, which I am sorry to find so much in vogue amongst you. I hear constantly of "taking strong positions and holding them," of "lines of retreat," and of "bases of supplies." Let us discard such ideas. The strongest position a soldier should desire to occupy is one from which he can most easily advance against the enemy. Let us study the probable lines of retreat of our opponents, and leave our own to take care of themselves. Let us look before us, and not behind. Success and glory are in the advance, disaster and shame lurk in the rear.

All this did was piss off his peers and subordinates, and worst of all offend Lee who more than any other general he fought seemed to want to smash Pope to prove a separate point.

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