Prior to the thoughts of a lot of experimental filmmakers (and, in this case, anime directors), storytelling has its universal Dos and Don'ts. Like plants need water, stories need at least some attention not to wither and die.
Take for instance context. If you have a very good story, then it can be understood without context. If you have a mediocre story, context is explained. If you have a low-rent spin-off of an established Japanese TV series, you basically just jump right in. Time is money, animators are expensive. Why waste screen time explaining what's going on to people that don't watch your TV show? It's not as if it the attention to quality in a movie couldn't, say, bring in new viewers.
Continue reading: Inuyasha: The Castle Beyond The Looking Glass Review