It was efficient as a medical transport between air fields, but it was unable to operate in remote, unimproved areas.
The Army needed an aircraft able to land at or near the site of a crash or battlefield for rapid cusualty evacuation and the C-15A was not able to perform this role.
The Army Air Corps tried for several years in the late 1920s and 1930s to get specialized crash recovery aircraft built, but all attempts failed. The primary reason was lack of funding so rather than sacrifice bomber and pursuit aircraft purchases, the Air Corps settled for conversions of existing aircraft, mostly in the cargo series.