I think CAREFREE was a deliberate attempt by RKO to change the Fred/Ginger formula. The team had fallen from 3rd at the box-office in 1936 to 7th in 1937, so clearly the public seemed interested in something new. Ginger was also notorious for wanting to break free of the team, and CAREFREE allowed her to be much more a comedienne separately than glued to Fred.
The attempt, however, wasn't successful. Neither CAREFREE nor THE STORY OF VERNON AND IRENE CASTLE ranked among the top grosses of their respective years.