Honestly, I think it's burn-out.
From what I've experienced personally, it can stem from various things
- Not getting recognition: fans that leave comments besides hearts, single digit views and likes
- Obsessively thinking about it and working on it daily (I think this is number #1 problem)
And then it turns into a negative circle that you can't escape. You think about it every day, you work on it every day, you upload things on a weekly schedule or more frequently and get little to no return.
A good analogy is something I just told a friend of mine, it's like a restaurant. There's a long-time eatery closing in my home town and everyone's reacting with "aww that's so sad! I love that place!" and the owner literally is responding with "clearly you don't cause you don't come in and spend your money here". Be it comic or novel, or some other endeavour, if you constantly don't get any responses until you finally pull the plug - that's too late. Now, you can be a fan and go off and complain that your "favorite" thing ended, but if it's not supported, that's what happens.
iirc another example close to comic creator's home should be the mangaka for Yami no Matsuei, as the story goes so far as I've heard/recall: the mangaka suffered an injury of some sort, healed, came back to draw, and their drawing style went from a certain stylistic style many were familiar with (and could attribute to the late 90s and early 2000s, though I see it as gorgeous), and it transformed into something more realistic; THE FANS did not like this change, did not support this change, and stopped purchasing the new releases, so the manga was pulled and ended before the actual plot ending.
For my own personal experience, what makes me quit a novel (writing it) is that I'm just not creatively flowing with it any more. That's 1, or 2, I have ideas for other stories. And 3, I have multi-plot ideas that would segway the plot into different directions and I'm unsure which would be "best" to follow, and end up doing none, and just ending Cold. Turkey.
I have thought more than once to end Nearly There Nicely simply because of the lack of attention it seems to get from readers. Especially after my car accident in May 2017, I was deciding to adjust my mode into just editing what I already had and "just write to 90" so I could edit that, and end it at 90 cold turkey. It felt like no one was reading it, no one was enjoying it, no one was liking it, and no one was even seeing it: I was invisible. Now, don't get my negativity wrong, I still feel highly invisible on Tapas. The only change is a visual record of views and likes. I have at least two people hitting like on the updates. I do preach that "you should write for you" but if you (being me) aren't happy with what you have, either quit or do something to make it better. I happened to find Tapas around November or something last year, and thought "eh it'll probably be dead like everywhere else." My first month here hit 139 views, for just the first 10 chapters (and the 10 chapters I consider the weakest).
Writing, creating, comic drawing, drawing, cosplay sewing - anything like that - it's all a very isolating creation. If you live in an area like I do that's super rural and doesn't have much of anything going on, then you will be pressed finding ways to stay motivated. Every few days still I think about carting it all up and quitting. I can only explain that this urge comes from the daily illusion of "no one sees my stuff". I think about it daily, I work on it daily; How come daily it doesn't have a number of views that matches how many times I obsessively refresh the page? That obsession of focus, without taking any breaks from it, can lead to the depressive lows. That's why it's good to have focus, but just as important to allow your brain to focus on something else - some movie, tv shows, your pets, gardening, something that just isn't "omg my character is cooking so I need to cook what they're cooking omg I must make it perfect because they do!" and then you curl up on the floor of the kitchen because you burned your pan to a crisp. It'll be fine- just soak it and see if you can save it. There, there, hypothetical me, just don't leave the burners unattended.