Simple. War leads to expansion and Custer had some luck.

So prior to the American Civil War(ACW) the United States had traditionally kept a small professional Army. To put it in perspective, “By 1845, the United States had about 7,300 men under arms to protect a nation of nearly 20 million people and 1.8 million square miles of territory.” Only in times of war did the United States expand the military, mostly as state militias, which were not a part of the regular military structure like the National Guard is these days, and volunteer units were organized, usually on time limited contracts.

So to lead these men, you need officers, which either came from the states themselves, and were usually non-military men, or from the United States Military Academy. There’s only one problem, West Point was not the large school it is today where it produces close to a thousand officers a year. “Between its founding in 1802 and the beginning of the Civil War in 1861 the U.S. Military Academy graduated 1,966 cadets, about half of whom served in the Mexican War.” As you can see, there weren’t enough professional officers to spread throughout the army if it was to rapidly expand.

With the outbreak of war, Lincoln called for 75,000 volunteers, with Congress soon authorizing a army of 500,000 soldiers. As previously mentioned, militias could provide some officers but they weren’t trained in the military arts. As such, the Army graduated Custer’s West Point Class of ‘62 a year early and sent the young officers to drill the new volunteers. After the army left the drill field and went to battle, Custer had the fortune of being in the right place at the right time. He carried messages for General Scott to General McDowell during the First Battle of Bull Run and then during the Peninsula Campaign and Maryland Campaign of ‘62 he served on the staff of General McClellan as an aid. In 1863 he was the aid of the commander of the Cavalry Corps of the Army of the Potomac as the Army reshuffled commanders and various level before Gettysburg, Custer was picked and promoted to Brigadier General OF VOLUNTEERS due to his aggressive nature and willingness to fight. I capitalized those words to note a distinction. Cluster’s rank was only to command volunteers. After the battle he was given brevet rank of major in the regular army and gradually made his way up to brevet major general and the major general of volunteers. After the war was over and the volunteers left the service, he lost his rank due to downsizing, took a leave of absence for a few months, and then came back as a lieutenant colonel in the 7th Cavalry.

Sources: The Regular Army Before the Civil War, 1845-1860 by C.R. Newell

Custer Victorious Gregory J. W. Urwin

Of Duty Well and Faithfully Done: A History of the Regular Army in the Civil War by C.R. Shrader and C. R. Newell