I could be mistaken, but for example two of the top series on canvas (by likes) are Sarah's scribles (1st spot on likes) and Brutally Honest. Those are short 4 or 5 panels style , black and white random/slice of life humor episodes. I don't think webtoons staff cares about that format, they give preference to 40+ panels per episode series telling a long story. Now, maybe webtoon did offer them and they rejected, idk for sure.
There's another series "Lost in Translation", that Is gonna be featured on Original, but it still show up on canvas (number 3 by likes on canvas). with a message "Error. This story has been removed from webtoon canvas, and will be re-launched as a webtoon original". So...while it has been """removed""" it is still occupying the 3rd spot, taking that spot away from all the series that ARE active on canvas, that fact alone should tell you how Webtoons couldn't give a shit about canvas.