a major part of the reason for this was that when a regular army officer was used to command volunteers, he still retained his old rank and position in the regular army. So when most of the field officers in the infantry regiments where made generals of volunteers, they where not replaced.
To quote August Meyers:
Aside from the few who had resigned to join the Confederate army, nearly all were advanced in rank; colonels and majors and some captains became brigadier-generals or colonels of volunteers. The twelve new regiments added to the regular army absorbed many of our captains and first lieutenants who gained a step in rank by the transfer. The lower grades were filled mainly by civilian appointments, many of them through influence more than any adaptability for a military life, as was demonstrated later on. The Government began to make some appointments from the ranks and later on increased them. These men, appointed from the ranks, as a rule made efficient and reliable officers, whom the rank and file could respect. Dixon S. Miles became colonel of my regiment and remained so until he was killed at Harper's Ferry in 1862, but we never saw him, as he had a higher volunteer rank. All of our former field officers were promoted and replaced by others, some of whom we never saw. The regular army, small in numbers, was stripped of many of its best officers. All through the war, companies were largely in command of first lieutenants and regiments were often commanded by senior captains....
Bringing the