During the time of the Civil War, weapon design was changing rapidly. I suspect many guns were made and discarded because a new design came out that was better than what was made even just short years or months before.
As was suggested, a lot may have depended on location. News of new developments and the manufacture and transportation of such would be problematic and take considerable time and resources. Even now, many people use what is available rather than wait for"the perfect weapon".
I many cases, all a particular weapon had to be was available and at least marginally adequate, with the emphasis on available. Like has been said many times on this site, the weapon you carry is far better than the perfect weapon sitting in the safe at home.
Wars tend to force development of weapons. This is often why civilians tend to buy weapons that the militaries have used. They must have been considered adequate since the army was using them. They also tend to be available.
That said, there is always a push for something new. Maybe the next super small weapon will be the deadliest thing ever created.