All military powers seem somewhat backward to civilians sometimes. They usually don't change everything every few years.
Another little detail about the .44-40 is that it was not available in the Colt Single Action Army until 1878, a few years after the army had adopted a different cartridge. About 20% of pre-war production was in that caliber, while nearly half were in .45 Colt. Available also were .44 rimfire and .44 Russian along with over two dozen other chamberings, some made only in very small numbers.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.