For some reason, the writers and editors chose to ignore J'Onn's non-Supermanque powers like telepathy, invisibility, and shape-shifting, and just had him using the standard flight, strength, invulnerability, and Martian Breath by the early 60s. This, I think, contributed heavily to the demise of his own feature and his leaving the Justice League. If he was nothing but an imitation Superman, why was he necessary? Who buys Hydrox when Oreo is available? Maybe he filled a niche in the earliest days of the Justice League, when Superman and Batman were little more than honorary members, but once Superman took an active role in the group, J'Onn's status became painfully obvious.