A lot of the anime I've watched are single, double or triple season anime series like Durarara!, Code Geass, Hagure Yuusha no Aesthetica, and Nabari no Ou, just to name a few. But there are also shows like Bleach, Naruto, D Grey Man, and Katekyo Hitman Reborn that have 300 or more episodes.
Why is that? How is it decided that these series get to go on so long?