"The story goes that Marvel (or Atlas, at the time) publisher Martin Goodman wanted Stan Lee to come up with a super-hero team comic because JUSTICE LEAGUE was a hit. But, being the loose cannons they were, Stan and Jack Kirby developed a new title that broke a lot of genre conventions. No costumes or secret identities, a lot of agita and in-group squabbling, more physical brawling than the DC style."
Also, DC actually distributed Marvel/Atlas comics and they had an agreement that Marvel wouldn't produce superhero titles that would compete with DC's titles. So, the "no costumes or secret identities" was a necessity.