Mainly because of superior Generals, like Johnston, and Jackson at first Manassas, and Lee at Second Manassas.
Joseph E. Johnston and P.G.T. Beauregard
Confederate Joseph E. Johnston perhaps does not receive all the credit he is due based on his performance in the US Civil War. He realized that his forward position at Manassas, exposed his army to a turning movement by Union forces. With Jefferson Davis' permission, Johnston began to withdraw from the Washington DC vicinity and move southward towards Fredericksburg Virginia. He also removed the blockade of the Potomac River, believing its continuation could not last. As an aside, US President Lincoln had some satisfaction at Johnston's retreat. McClellan moved troops south to take over the vacated Manassas.
Gen. Robert Patterson wasn't in the First Bull Run, staying idle in Winchester, being baffled by the Confederate Gen. Joseph Eggleston Johnston, who silently retired the troops before him, leading them by train in Manassas
For the Union, McDowell. For the Confederates, Joe Johnston and P.G.T. Beauregard.
Stonwall Jackson.
The South had several generals that were better than their counterparts. Stonewall Jackson, PT Beauregarde, and Joe Johnston all won fame in the war, and their handling of their troops won the battle.
Lots of Confederate and Union troops alike
At the First Battle of Manassas Gen. Joseph Eggleston Johnston had the overall command of the Confederate Army, while Gen. Pierre Gustave Toutant Beauregard was left the tactical direction of the battle. At the Second Battle of Manassas the Confederate leader was Gen. Robert E. Lee.
The Confederates were commanded at First Manassas by General Joseph E. Johnston and Brigadier General P.G.T. Beauregard on July 21, 1861. At the Second Battle of Manassas the commander was General Robert E. Lee on August 28 - 30, 1862.
General Irvin McDowell who lost the battle
By February of 1862, the Union's general in chief, George B. McClellan had amassed a huge army that endangered Confederate Joseph Johnston's army in northern Virginia. McClellan's Army of the Potomac was estimated to have over 150,000 troops. Johnston's army was only one third of McClellan's. Johnston therefore ordered his forces south and to consolidate on the line of the Rapidan and Rappahannock rivers.