Much like Katsura Hoshino's manga on which it is based, the D.Gray-man anime was extremely popular when it started airing in 2006. As was the case with Naruto, One Piece and other shonen series of the early 2000s, D.Gray-man had length on its side, and surpassed 100 episodes. Despite its long-running tenure, the series abruptly ended even as the manga continued.
Source：https://www.cbr.com/what-happened-to-d-gray-man-anime/
