1. Mangaka decided to drop it
2. Publisher decided to drop it
3. Reader don't like it. Or get bore with the story
Even if it great. If the jap reader bore with it. It will be dropped
Jap manga always release in manga magazine. Be it monthly, bi monthly or anything. They always do polling. Lowest manga rank will have warning, if their rating don't increase, mangaka will be put to stop the manga. It happened a lot. Sadly, the polling is only for jap readers. In my favorite manga case, this manga that have gone world wide & have anime, and fanbase, even put to stop because the jap readers get bore with it (it's adventure comedy). But we, outside jap readers who really like the manga and didn't want it to stop have to accept that our manga is forced to end because the polling that held for jap reader.
I think because they release every single chapter sometimes people lost the interest because it too short. Let say, we prefer to read harry potter if it release in 1 book even if we have to wait years to read it, rather than we read every single chapter every month and we forget what previous chapter tell.
Or People don't buy it and mangaka decided to drop. I bet 80% of people who read it illegally online like this doesn't buy the manga thus not supporting the mangaka to continue financially. Remember to buy the book if you like the manga.
Sometimes i really don't get japanese editors or mangaka. WHY DO THEY ALWAYS DROP AMAZING STORIES?!? ╥﹏╥