My personal theory is that a lot of webcomic artists, while talented artists and writers, are not necessarily good COMIC writers, which is a very different skill, and that attempting to tell a written prose-style story through the medium of comics takes FOREVER.
Let me use some examples:
This is the opening of Matt Fraction's famous run on Hawkeye:
This single page does a LOT of work. It shows an action sequence of Hawkeye falling, it uses naturalistic dialogue to introduce a core theme of the series: Hawkeye's inferiority complex being "The arrows guy" on the Avengers. It thematically grounds this series in comparison to other superhero comics, the first thing our hero does is fall off a building and break a bunch of his bones, then end up in an ordinary hospital instead of some sci-fi bacta tank thing. The Art isn't just "Holding" the dialogue, it's doing the weight of the storytelling.
This is also 1 of 20 pages of this issue. This is 5% of a complete, if small, story. The actual plot points of this issue are pretty simple and small, you could probably sum up "What happens" in a few sentences if you wanted to, but it feels like a lot more happens because the dialogue and art are working together to craft a story in a way beyond just advancing the plot.
Hawkeye #1 is very much a Comic story written to be told in comic form, and it shows. A big thing this page does is that the action (Clint falling) and Clint's monologue about his insecurities don't compete with one another for space. That's the sort of thing you can do in a comic.
This is also being done by an industry professional team, with different people doing art, colors, letters, writing, editing, and more all as a full-time job to produce a 20 page comic in a month.
I think a lot of webcomic artists don't think of themselves as writing a comic story. They think of themselves a writing a novel, or even a series of novels, and using a comic as their medium, not fully grasping just how little novel-style plot can be advanced in a single page of comic.
Look at this page of Girl Genius:
This page is basically a script with some pictures. The plot advancement and dialogue is the key. It's a full page of comic, but...not actually that much happens. The two characters say these words to each other. If you are charting out the plot of Girl Genius Volume 2, this is PART of the bullet point that reads something like this
"Agatha wakes up in a strange place with Moloch Von Zinzer, the soldier who stole her locket in volume 1. He explains that they are prisoners of Baron Wulfenbach, who believes Von Zinzer to be a spark and that Agatha is his lover/assistant. The two of them agree to maintain the charade until they can figure out a way to escape".
That takes about five or six pages, two weeks of full-page comics in a thrice-weekly update schedule. some other details are established as well, but from a "plot points getting checked off "standpoint, that's what happens in those pages.
And, while a lot can be said about Girl Genius's pacing, the Foglios are pretty good at writing long-form comic stories. They know the medium of comics quite well, their main issue is that they are trying to use comics to tell a story that contains a number of discrete plot points best suited to a series of novels.
I think a lot of webcomics have a grand story in mind, but it's written as a series of exciting and interesting plot points, and they severely underestimate how slowly comics cover complex plot points, even if you just have two characters stand there talking to one another, you've still got to draw a whole comic's worth of art for that. If you're a webcomic doing regular releases, then "Two characters have a slightly long conversation" can eat up months worth of updates during which nothing really happens except that conversation. Each plot point on your story outline can take, at minimum, three pages to fully cover.
And if you're an artist really excited to tell your story, spending that much time and effort to get to the "Good stuff" because "The characters sit down to a meal and talk about their plans" takes fifteen pages to do, it's easy to get bored of the story you are in the middle of telling and have the urge to jump off to tell whatever excites you at the moment.
