Thanks for the clarifications Bob. Though M&H did, as I said, make guns for the 44 S&W American (at least according to Barnes) in addition to 44 M&H. I did not mean to imply that the M&H was adopted by the military.
Now is the ops question answered? Why the 45 Colt rather than a 44 caliber for the militaries sidearm?
It's clear why they were not interested in the 44-40 in a long gun.