Theres a fine line between film franchises and manga. With film, you have more opportunities to play with the rules you already set in previous installments. This is how film industries can get away with multiple spin offs of a particular series. We'll use Star Wars as the example again. The original star wars trilogy was about Luke Skywalker and his epic adventure. The prequels, as much as they sucked ass in relevance to the story, were their own adventure story in their own rights. Because the first 3 films only told the story of one man in an entire universe of stories, spin offs can exploit this, introducing new characters or backstories to previously known characters. Typically, if a film makes money, someone will come along and buy the rights to the property so they can make money.
Manga is a bit different. Unless a manga clearly says that there are many different stories to be told (I cant think of any off the top of my head that fit this description), then a manga only focuses on one story. We'll use Naruto as the example here. If the plot of the series didnt revolve around Naruto, and instead Naruto was just a part of the plot (its a bit tricky, but theres a difference. Think of Star Wars again) then the writer could get away with spin offs and backstories. However, because Naruto IS the plot, as soon as his adventure ends, the story ends with it. The only way to continue the story is to focus it on another character, but it wont/cant be of the same story. It would have to be a spin off of some sort, and again, because the story is wrapped around Naruto himself the spin off would HAVE to be an original story that follows the events of the main plot. And if that's the case, then it would simply feel like Kishimoto simply cant give up his story and is just prolonging the day where he has to say goodbye to his characters (besides, if kishmoto really wanted to give character backstories he would have spent more time developing his side characters)
