Manga can be extremely fast paced with releases, often weekly, meaning artists/writers need to be spamming their own works to the point of regular burnout and health problems.

If a project is doing very well for popularity and money then there's going to be pressure to not only keep it publishing constantly but also to keep it going to maintain that money. This leads to a lot of "filler" before actual filler even gets brought up simply to stretch out the story for far longer than it should run for.