The FM series were often used by professionals because they were smaller and lighter than the F series, they'd now be termed Prosumer I guess.

Where 35mm was the only format used then the F series was the better choice, but many professionlas shot mainly RF and LF with only a small amount of 35mm, and the FM's were ideal, specialist Nikon lenses were (still are) easy to hire if needed. FM's were alo used as back up cameras becuase of there much lower cost new.