Captain Marvel has always been a hardluck case in the copyright arena. When he was first developed by Fawcett he was going to be "Captain Thunder" in "Flash Comics" or "Thrill Comics". But Fawcett was beaten to the punch by other comics using those names so they had to settle on "Captain Marvel" and "Whiz Comics". Marvel staked a claim on "Marvel" back in the late thirties with "Marvel Mystery Comics", so once the Captain Marvel copyright lapsed they could claim that only Marvel could use "Marvel" as a comic's name. Which is why the Shazam
comic of the seventies had to drop the "Captain Marvel" tagline on it's covers.