I think it is because the anime wasn't cut short and didn't get a proper ending, the anime didn't tie up things nicely, the anime was made while the manga was ongoing and now they'd like those seasons to be animated, etc.
Ranma 1/2 had fans asking for more because the ending came falling short of the manga and it wasn't conclusive, that being said, Rumiko stories rarely have tight endings.
Ruroni Kenshin stopped some seasons before the last chapters of the manga, some OVas came after but they butchered some aspects to the point that not even the author liked them, to my understanding.