Remember that M-1s went mainly to the Army and the Marines had to make do with 1903 Springfields in 1942-43. 

Four uses for Krags come to mind: 

The first being USCG beach patrols, mostly begun in 1942.  On the Outer Banks and on the remote stretches of northern California, Oregon & Washington, the beach patrols rode horses. A Krag carbine or cut-down rifle would be well suited to such activity.

The second being the arming of the YPs, but that again was mostly in 1942.  YPs were large yachts (mostly in the Atlantic) and large fishing boats (North Pacific & Alaska) used as advance warning pickets and anti-sub patrols.  The YPs went into water where sea ice (and maybe polar bears) might be encountered.  Better to have a Krag with 220 grain bullets on board than a Springfield with M-2 Ball.

The third use might be for guard duty or even for guards at industrial sites. 

A fourth use might be for drill rifles in recruit training.  In Boot Camp in early 1960, I was issued M-1903 Springfield SN 4003.

All of the above are just guesses