In those days it was still assumed that comics were read almost exclusively by young boys, and that they would stop reading when they got old enough to be interested in cars and girls. They figured that a person's comics-reading life was, at most, 5 years (and their attention span shorter than that), so continuity was only worried about in the short term. They felt free to retcon anything they wanted and even recycle entire stories almost scene-for-scene.
The obsession with canon and continuity didn't begin until geek culture started going mainstream in the 70's and 80's.