Had they pushed forward at once they would have unquestionably captured Richmond.

But McClellan's fault was over-caution, and he believed himself opposed by a very much larger force than that under the command of Magruder; consequently, instead of making an attack at once, he began regular siege operations against the works on Warwick Creek and those at Yorktown.
The delay saved Richmond.

Every day re-enforcements arrived, and by the time that McClellan's army, over 100,000 strong, had erected their batteries and got their heavy guns into position, Magruder had been re-enforced by some 10,000 men under General Johnston, who now assumed the command, while other divisions were hurrying up from Northern and Western Virginia.

Upon the very night before the batteries were ready to open, the Confederates evacuated their positions and fell back, carrying with them all their guns and stores to the Chickahominy River, which ran almost across the peninsula at a distance of six miles only from Richmond.
The Confederates crossed and broke down the bridges, and prepared to make another stand.

The disappointment of the Federals was great.