>>118758301A. Wide appeal often requires your story to be typical enough for the audience to not be put off but also have enough threads/strange qualities to stand apart.
B. The comics that tend to get popular are still adding characters/threads up to 100 pages in.
C. As an unrestrained forum, the internet does not stop creators from making a comic that exceeds their actual grasp.
D. Giving each page a punchline or revelation helps popularity, but also harms pacing and requires more pages in order to actually convey information and close the loop on any surprises.
E. As an unrestrained forum, the internet allows amateurs to post without any supervision or preparation.
F. With no guarantee of success or even stability, webcomics are the domain of hobbyists, failures and amateurs. People with skill and experience tend to find print work.
Combine all of the above and almost every webcomic competent enough to gain a following is authored by someone who hasn't properly planned and constructed their book, leading to it going far longer than would be reasonable, leading to the comic ending prematurely when they either burn out or need to spend their time making money.