In ALL-STAR COMICS? Black Canary came along after the war, I think 1948 or so. She started as a character in the Johnny Thunder strip in FLASH COMICS, then took over the strip and took Johnny's spot in the Justice Society. (That makes her sound a bit cut-throat, but it was just trends).
Wonder Woman was given a lot of support as a character. She was introduced in a special 8-page section of ALL-STAR COMICS (not part of the story) before being given her own title. Most likely, she was great for merchandising. Even when her own comic didn't sell in good numbers, her image made money on lunchboxes, toys, decals, Halloween costumes, that sort of thing. Many characters have more more profit through merchandise than the actual comics ever did.