Because It Came Up

Lt. Col. George Armstrong Custer's sidearm.

Well at The Battle of the Greasy Grass anyway, he had numerous sidearms.

I vote for SAA because the Army was on a standardization kick at the time.  At least one of the 7th Cavalry troops lobbied hard to retain their Spencers to no avail when the transition to the M1873 Flop-Top carbine happened.

Of course, being a contrary cuss, a standardization kick would trigger a response from the Brevet General to use anything but the standard weapons.  He was packing a Remington rolling-block in .50-90, for example, rather than a Trap-Door.