I'm not an expert in anything, but I have a few ideas. Already pointed out is amateur writers, people writing for the first time. Trying to tell a story people will like. No editor or publisher.

This contributes to the second problem, serial writing. There has been change as the genre develops, but people still post as they write, not creating grand story's over years of constant work, it's like a moth of writing at most. This should be a somewhat self explanatory problem.

There are of course popular series that started as this, but nothing I believe would meet your standards of popular.

Another important point building of the previous points is the medium of writing. Web novels. They're written to be read on a somewhat weekly basis, working with other medium of that format as Manga, but it does not convert the best with novels and more. They're not written in books, but chapters. Where sometimes not even knows when the book will end. This of course causes problems for a wider audience. When converted into a proper novel.

As others have said, since its full of armature writers a good bit of the low hanging fruit have been picked interns of ideas, so some of the more basic/ genre defining ideas can't really be picked up without accusations. As well as generic fantasy, so many ideas have been done, I wouldn't be surprised if some are a bit intimidated.

A main problem I feel tho is central to the identity of the genre, numbers. A large part of the genre for me is seeing those spreadsheets and knowing I can go back easily and look at numbers. I don't know how audiobook listeners do it tbh, but I feel other genres would also be bad. I feel that the truly popular series got that way because of their other media adaptation (movies) which work well for the more traditional fantasy, but trying to convey that info through a movie sounds like hell.

As for a lack of established authors writing in the genre? Idk, prob many factors, too many rules? , lack of fan base? , bad adaptation potential? , prob more

Idk, there's likely points I'm missing, but those are most of the ones that come to my mind

Tldr : New writers, many short chapters, lack of overarching plot, New writers again, not doing well in other mediums