Brother,
You are absolutely right, it was a backwards step in design. The official reasons were cost of the ladder sights and the simpler two leaf sight of the '61 were considered more than adequate in an Infantry Rifle.
OK, I think I'm going to get into trouble with some of my American bretheren, but it is far better to learn the facts and mistakes of history, so we don't keep making them.
The REAL reasons were unlike the British Army who took decisive action on lessons learned in the AWI and developed the Baker Rifle (which was far superior to any military rifle we had until the M1855), the American Army Generals never learned to think outside the box and effectively use Riflemen.
The tragedy was that after the M1855 came out, forward thinking Army Officers in the West actually TRAINED the troops in using the Ladder Sight and had them PRACTICE marksmanship to the point they were effectively engaging hostile Native Americans at distances out to and including 700 yards. I have not been able to document whether they were copying how the British Army was already training British Soldiers on Military Rifle Ranges with the P 1853 Rifle Musket when the '55 Springfield came out, or if the American Officers in the west came up with it on their own. (Somewhere in my library, I have a daguerreotype of a British Army Company training at one of the English Military Ranges in 1856.)
When our own UnCivil War broke out, there was almost NO training whatsoever in Marksmanship with the '61 Springfield. This even after the initial rushing to get troops into the war and when they actually had time to do it. American Generals on both sides continued Napoleonic Tactics and almost none of them ever got near the accuracy out of their Rifle Muskets that was possible, with the exception of Berdan's Sharpshooters. (Yes, I know they usually used different rifles, but with ladder or other better sights.)
I have almost desperately searched for historic documentation that someone on either side came up with some of even the most basic marksmanship training or techniques for the standard Regiments, but have never found any. Of course with the limited resources of the South, it is somewhat understandable, but the Yankee Army was always almost swimming in supplies and funding and they almost never did it with the exception of the Sharpshooters.
Fraternally yours,
Gus