Same reason a Koby and to add a few other animes to his list: Kekkaishi, Gantz and D. Gray-man. These are mangas that were turned into anime which never completed adaptation of the entire manga. However, D. Gray-man and Gantz are still ongoing mangas The anime adaptation of D. Gray-man and Kekkaishi merely stopped midway and did not finish the series with a filler ending however Gantz did.