Anime series are sometimes original works, while most others are based on existing media, such as manga series or light novels. Over time, countless manga and light novel series, both major and obscure, have been adapted into wonderful anime series, but there is still more ground to cover. Plenty of manga have yet to get an official anime adaptation, including the short series. If the shorter series don't have enough material to become an anime series, then they could become an anime movie instead.
Source：https://www.cbr.com/manga-series-that-should-be-a-movie/
