Captain Marvel was perhaps the most popular superhero of the Golden Age, outselling even Superman and outlasting all but a few of his competitors.
Given Stan Lee's fondness for borrowing character names (and sometimes quite a bit more) from other, now defunct publishers. It's surprising that it took until 1967 for Marvel to come up with their own captain. It could be that Lee was reluctant because because the rights were still out there (DC would license the original big red cheese in the Seventies and make him a permanent part of their universe).
There is speculation that Marvel finally got around to creating their own version because a vanishingly obscure publisher put out their own Captain Marvel, an android alien (hey, at least they left out mutant) with one of the great goofy powers of all time.
From Carl Burgos (who had more notable things in his resume), here is one of the almost forgotten moments in comics history.