Main problem causing this might be, because there's an overlap between the manga and the anime. The latter being based on the former so it can't flow as quickly as it wants, lest it catch up with the source material, in which case they'd have to halt production for a while till another buffer has been created and with a once a month release date for a lot of mangas, such a buffer could easily comprise several years (Keep in mind 4 episodes a month for the anime vs 1 for the manga and in most cases 1 episode = 1 manga).
Pure anime's not based on manga, but which have the occassional one-shot derived from it, can focus more on the story and get it properly flowing. The latter are also often shorter in length than the hybrid versions like Naruto or Bleach.
Attraction issue? The longer something is running, the more likely that someone will accidentally stumble over it, from there it becomes a word of mouth situation. So even if you've maxed your current audience, by having the show run longer, you increase the chance of someone new coming in and potentially dragging in new viewers, this even taking into account loss of interest of previous fans.
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