It is the legendary japanese people trait, they know how to start a story, they never know how to end it.
It's not <japanese people trait>, honestly it's more like <asian people trait>, because chinese or korean authors do the same BS all the time when title is not being axed fast enough.
We have good start, solid background history, engaging story and than it goes 20-50-100 chapters forward and main story just derails into a million tiny threads, each of which continues the initial story in the same exact way - doing something completely different which are not even marginally close to what has been happening so far. Some characters appear even though they don't really contribute anything to the story in the long run apart from being temporary villains or filler side characters, others disappear without a word of explanation, or the entire story goes so sideways that larger parts of the previously created narrative and characters disappear. And we have a dissonance what has just happened.
Let's put it bluntly - with very few exceptions, the Asians are capable of creating very good worlds and plots for LNs or mangas, but then they can't for the life of them take a proper direction afterwards. We end up with a really refreshing and absorbing beginning after which we are eager for more and have high hopes for the title. And then a dozen or so chapters pass, during which the story somehow builds up, the plot progresses.... and then we hit a brick wall, because the biggest problem comes out - the lack of forward planning for more than 5 chapters ahead. Suddenly plots and characters totally disconnected from everything appear out of nowhere, suddenly completely unmentioned locations appear, suddenly the whole story jumps from place to place and the typical search for where to hook the next "arc". And then it's predictable - the author stops having any ideas that would tie in with the title, because he never even planned to get to this point, and either slows down with the release of subsequent chapters to the point where the next one comes out in like 5 months, or the chapters are so pitifully fragmented that you'd have to collect 15 of them to match the first chapters, or both, or the title falls into hiatus.
Sometimes multiple times because don't forget- HxH is a thing and rumor has it that soon we'll see ANOTHER "reboot". After previous 2 which ended with hiatus and whole manga got called "hiatus x hiatus" because people had enough of this bullshit.
And when a title has reached such a stage, well axing it is pretty much the only way out of the situation, because it has to be finished SOMEHOW, and the ideas have long since run out.
Probably the best example of how much of a difference forward planning and the lack of it makes is to compare the likes of One Piece, where the manga has reached over 1,000 chapters, the story is as engaging at the beginning as it is at the end, and all the preseted events are closely incorporated into world building and we can easily connect 2 seemingly separate stories. Now let's switch to let's say Tales of Demonds and Gods. Currently we're at 428.5, because fuck you and normal counting so if we count that all chapters from ~80th are split into two it would mean that's around 770-780 normal ones. And as the time goes next chapters are being shorter and shorter with less and less content and more and more useless talking on panels without any sense whatsoever. And as much as I personally like the story deep down I know that one day it will be axed.