Plot mostly.
Like with Blood Alone. Art declined dramatically too, but boring plot is the main reason to drop it.

Bad or boring plot. I won´t bother with a proper ongoing if only the art is good and I don´t mind if the art quality drops (fill-in artists became a plague in US comics) as long as I still know what I am looking at. HxH really pushed that and the "fixed" Chimera Ant arc in the tankoubon releases remains an issue in places. The plot 100% carried that manga but it can look good. Mobile Suit Gundam Thunderbolt´s art took a hit for about 2 volumes and never fully recovered due to the artist needing to switch his drawing hand and retraining himself as the manga went along. Neither of these ever stood a chance of being dropped though and HxH´s Jump releases from the 00s are about as bad as it gets! Look it up. Even the unfinished art of Historie in the magazine releases is nothing by comparison.

The so-far least-picked character death option can be a plus for me. Consequences and all that. JoJo Part 6 really picks up once the bodies hit the floor around the 60% mark and remains a blood bath till the end. Even my favorite character from the original universe bites the dust via head split. Metal AF.

Different answers for finishing vs catching up. Barrier to finishing a completed series? Bad or boring plot (including meandering plots! I hate it when a series drags on, esp when it started good). Barrier to catching up a series that's ongoing? Like falling out of an exercise routine, I missed a few chapters and never got back into it. Usually because the plot wasn't compelling enough to begin with but it doesn't mean it's bad.

Like the freakin Energizer bunny, it just keeps going on and on and on and never reaches any conclusion. Seriously HOW many times can one high school kid and his band of plucky friends save the world?!

Some other reason.
Mainly badly executed tropes and consistency. A good example of both would be Eleceed, it's so formulaic it hurts, chapters feel like a checklist destroying any hope of consistency and ruining many tropes.

It's mostly what keeps me from reading most shoujo and josei; I can't stand a the shoujo trope of making the lead intelligent done wrong; and most redeeming qualities for which I judge joseis are the conclusions, but it's really frustrating to see how inconsistent some are after lots of chapters (many tropes involve acting out of character). I mean, the deception hits me unusually hard on those genres.

I don't care how surreal a series is (I find trying to read the intentionality on those especially entertaining). I mean I've read some terrible series. For me, as long as there's some redeeming quality, it's enough to not drop it on the first chapter. If I get bored, but the series still has what I liked, I just write a review and leave it gathering dust on my lists ... Also many manga's have serious issues setting the scene, the tropes involved usually bother me more than a bad plot.

I will regularly drop a series for having bad writing, but I don't consider that a reason for not finishing ones I like but don't finish. I catch up, miss a few releases, life happens, I come back and think to myself "not yet, I'm not feeling it" and then I've not finished it. Same reason I wait over a year to watch an anime I bought that I ended up enjoying.

I'm hoping Pumpkin Scissors gets a proper conclusion before I go back to that one.

I just fall off for a bit then its hard to jump back into it. What do you mean I'm 100 chapters behind on One Piece?

Most of the case is "Like falling out of an exercise routine, I missed a few chapters and never got back into it." (Though, sometimes it's simply because the plot is boring.)

I usually put this series on hold in my mangaupdates list, like Akagami no Shirayukihime, The Ancient Magus' Bride, Ao no Exorcist, and D.Gray-Man for me. I promise myself to re-read it someday.

Some other reason

The main reason I quit stories nowadays is because they were just so unoriginal.

So many stories nowadays, feel like the author were just following a template, and did not bother to try to put any twist in the story.

Bad or boring plot / characters (I like character-driven stories) (lack of interest isn't always a quality issue) > fell out of routine / lack of access to next chapter or long hiatus / busy with real life, never got back to it

It's easier to have the mental energy for simpler plots, so good-but-complex series are more likely to fall victim to the latter.

I voted some other reason because I also sometimes stop reading a manga when there's only a few chapters left. The reasons vary. I'm more likely to leave it off too long (perhaps waiting for the 'right time' or expecting to recap myself), endings can be dissatisfying, and my attention often drops off before I'm finished something regardless.
Manga is among the easiest media for me to finish, though. If it's a video or non-linear, I'm very unlikely to get through it. (anime, webseries, visual novels, videogames)

Bad or boring plot, definitely.

my reason is that, sometimes i keep starting new manga when i already have a lot lol, then i forget to read my other manga, but i dont really forget plots much tho

rapid success in unlikely scenarios
you'll happily binge a dozen chapters of world building but then the MC becomes OP and you're sick and angry because there's nothing more to look forward to except more copypasta arcs

I’d say for me mostly when I fall off of a manga is because it’s not being scanlated anymore or taking forever to be scanlated.

I’d generally love to finish out a series especially if it’s been done for years, but nope. Scanlation groups fall off. 😞

It is usually combination of bad characters and bad plot. If I don't like the manga after reading first 20-30 chapters to if I like it or not.

But I often try to give it another chance, because, like if it is highly regarded as good manga and I don't like it I'm wondering if I'm missing something. But when reading it feels like chore I just stop reading it.

I get bored easily so short manga are my bread and butter. If i find one that’s really good and completely scanlated, exceptions and all-nighters can take place (like Chainsaw Man).
I was closely following Yoru no Uta because vampires but got sick of it. Even so, it’s prob one of the longest manga I’ve read sitting somewhere around 150(?) chaps.
I seriously don’t understand how people can read chapters upon chapters of the same thing lol

So, having bad characters or having a favorite character die should all be with the main problem, having a "bad or boring plot". Any problem with the story (besides bad grammar/translation), all has to do with having an issue with the plot.

Falling out of a routine/being busy with real life is pretty much the same thing in my opinion.

The first thing that will make me drop a series though is a drop in translation/grammar quality. I've seen this happen many times when a scan group drops it or the translator or proofreader changes. If I really like the series then I'll put it on hold and either wait until another group picks it up or it has an official release to buy it. If a year or two passes without either happening, then I'll drop it. But if it's not a series that's not worth the wait, then I won't bother.

The second important thing is if the plot is interesting enough to keep my attention. If the plot becomes stagnant, I'll give it a few more chapters to see where it goes. If I don't like it, then I won't finish it. ^^

I dont start something that I wont finish, I dont consider 1-2 volumes of a manga started when how many go over 20 or just start to get the plot laid out by volume 3.

what stops me from reading a manga/finishing it is always going to be inconsistent scanlations. how many I have where there are year log gaps in-between chapters, at that point it just gets lost in the pile.