Much like the manga that it was based on, the anime series, D.Gray-man was very popular when it started airing in 2006. As is the case with Naruto, One Piece and other shonen series, the show also had longevity on its side -- reaching over 100-episodes. But despite all this, the series abruptly ended, even as the manga continued.
Though it's still fondly remembered, at the time, this caused the high level of interest in D.Gray-man to plummet, which wasn't helped by the separate issues plaguing the manga's release. Here's the story behind the once-popular series, and how, bit by bit, it fell off of the radar.
Like many popular anime and JRPGs from the era, D. Gray-man has a decidedly religious undertone to its premise, though its use of Judeo-Christian theology is loose at best. The series is about a group of exorcists called the Black Order, who use an array of mystical weapons powered by "Innocence" to defend the world against the Noah family. This family is made up of corrupted reincarnations of the Biblical Noah and his followers, who, led by the Millennium Earl, wage war against God and His creations. The Noah family use Dark Matter to combat the Black Order's Innocence, as well as a means to summon and control demons.
The central protagonist in the 19th-Century-set series is Allen Walker, a new, teen recruit of the Black Order who has trouble controlling his own mysterious -- and dark -- power. The original series adapts approximately 210 or so chapters of the manga, beginning initially in an episodic, monster-of-the-week format.
Her visually striking, gothic art style, interesting characters and elaborately detailed action sequences have earned creator Katsura Hoshino high critical praise. This is reflected in the sales figures for the manga, which has made the New York Times Bestsellers list in the past and, to this day, still has 24 million copies in circulation in Japan. With stylized action and an ever-popular manga to adapt, why did the anime series suddenly end, then?
The anime series lasted for 103 episodes, spread across three "stages," or four seasons. After the conclusion of the fourth season, however, the series simply stopped. It did have an ending of sorts, with the Black Order's future left uncertain, however, this wasn't meant to be the end of the series in general. In fact, even the season's conclusion reignited Allen's determination to defeat the still-alive Millennium Earl. With the manga continuing for a long time thereafter, why didn't the anime continue to adapt it?
The biggest reason was pacing. The anime already had to create excessive amounts of filler storylines so as to give the manga time to proceed much further, giving the anime something to actually adapt. Ironically, some of this filler actually contradicted certain events and elements of the manga. While the normal season breaks and a steady amount of filler might have allowed it to be evenly paced with the manga, as is the case with other popular anime, that wasn't possible with the D.Gray-man manga.
Katsura Hoshino was sadly forced to put her manga on hiatus numerous times for health reasons: once for a case of norovirus, and another time due to a neck injury. This kept new chapters from being published regularly, and when it did return to normal publication, it shifted serialization from Weekly Shonen Jump to the quarterly Jump Square Crown.
The anime eventually returned in the 2016 sequel series, D.Gray-man: Hallow. This TV series, initially suspected to be a reboot, instead adapted the manga from around where the original anime had ended. Unfortunately, Hallow had the opposite problem that the first anime ran into. Whereas that series ran out of material adapt, Hallow instead rushed and paraphrased much of the new chapters' material. Hoshino, who was initially hesitant in creating Hallow, was also critical of it. It was perhaps this lack of support for the adaptation that eventually saw the DVD releases for it canceled for "various reasons."
The manga series is still going strong, and while some feel that it may finally be nearing its end, many fans believe that it has the potential to last for years more, much like contemporaries One Piece and, to an extent, Naruto. Sadly, it doesn't seem that televised D.Gray-man adaptation will have the same perpetuity, which has been an inarguable cause for its downfall in popularity.