Specifically for Rumiko Takahashi's works, studios begin adapting the manga too early and they run out of canonical content.
Inuyasha got finished with Final Act, but it was 5 years after the original run.
Ranma manga ended 4 years after the anime ended.
Blame Rumiko Takahashi, she writes too slow.
Edit: I was going to write out a serious post drawing parallelisms between Ranma and Inuyasha and their anime runs and where they started to create filler content. But I think ill save it and make a mega post some day.