In most cases, the explanation is as follows:
Beginnings tend to be stronger because any series -- manga or otherwise -- has to gather its audience. In mass-market work, for significant profit, that generally has to be a pretty significant hook. It requires a lot more planning, usually, because you haven't hit a stride yet and can't just do one adventure after another with the same cast of characters you may not have introduced yet.
Middles tend to have mix-ups, escalations, and complications as both the writer tries to keep interest in doing the series and keep the audience's interest as well.
Endings are a very, very complicated matter. Sometimes it's because a series is cancelled with little notice, sometimes it's due to editorial mandate or interference, sometimes it's due to an author having no idea about how to resolve the story, sometimes it's just out of sheer desperation! Most stories, I would say, the author doesn't know exactly how they want to end it. They may have some idea, but until it's presented to them and they know it's here, they don't have to think about it. It is extremely rare for any writer to have a series planned out beginning, middle, and end from the start, because most writers who would do a lengthy series want to do it professionally -- making a profit from it -- and not just to tell the story. It's just not practical to plan out a whole saga that you may never get the chance to do, when you could use that time to develop a variety of ideas.
In general, whether you're doing a series for an editor and/or publisher, or if you're independently putting out a series, there are still other considerations. For example, burnout is one of the most prevalent dangers any creator faces in a creative career. Even when you find success with a story, sometimes you get really really tired of it, and by the time you've decided to bring it to an end, you can barely endure working on it. And whether ending a series is your decision or someone else's, if you had some ideas for the ending and find yourself suddenly faced with the need to end a story, you might think it's a good idea -- you won't get the chance again with this story! -- to just cram in everything you thought about.
Ultimately, endings are not generally done particularly well, even if one distills it down to a purely technical standpoint. It's especially difficult to take a story, especially one that has gone on for some time, and bring it to a singular close without failing in some way to be satisfying.
I personally feel like it's usually a mistake to try and regard an ending as a distinct moment and approach it as a singular thing; I feel nowadays that it's typically preferable to handle an ending almost as incidental, an opportunity to touch upon things you may have neglected to address before then -- because you aren't likely to have another opportunity after the story ends -- but not something you should feel you have to cram literally everything into and make it super special and super drama and whatnot.
But I'd always rather see an open ending that lets you think the heroes just went on as before and kept having their adventures. I've never been fond of "the magic goes away and LOL we're okay with it!" type writing, and I've likewise never enjoyed endings that try far too hard to tie up every single thing neatly and pair people off, etc. I'd rather be a little disappointed at an ending that doesn't resolve enough than be absolutely devastated at a terrible ending.
A bad ending can ruin all of the enjoyment that came before it by tinting everything that happened. It returns as a bad memory and makes it hard to enjoy the story otherwise, knowing that's looming on the horizon. It's a little easier if it's a separate book in a series -- you can just ignore the book and never read it -- but it's a lot harder when it's different. Though fortunately it's usually chapter-based in comics, so you can usually still jump off before it enters the final story that ruined it.
