I only watched based on translating the subtitles, but I want to offer my insight based on what I observed. In the West, Takehiko Inoue sensei is known more for Vagabond. He is usually regarded a master in manga. Slam Dunk is old, in both anime and manga. First, the anime. Even for manga readers now, it is difficult to consume the anime. The anime is very outdated and there are no benefits in watching the anime instead of the manga, unless you are watching for nostalgia. So people choose to read the manga instead. Second, it is difficult to read the Slam Dunk manga. One reason may be because of the beginning art style, but mostly because it is not available online. Of course this is an illegal method, but a lot of popular manga in the West become popular because they were consumed illegally online, and eventually boosted its fame through social media hype, enough for people to purchase the manga. Slam Dunk also has an illegal scan online translated to english, but the scan quality is not nice. It ruins the reading experience, especially because a lot of the readers want to experience Takehiko Inoue's art. And I have seen many manga readers stop reading Slam Dunk online because the old english translations are bad. Overall, I think Slam Dunk anime is outdated, and the manga is inaccessible. I also think the reason the movie was not a success, is because in the foreign anime community, there is still a negative stigma with using 3D/CG. A lot of people had negative reactions when the trailers were revealed to be in 3D/CG. Even though Takehiko Inoue has many fans in the West because of Vagabond, I think many fans feel like they wouldn't be able to see the art they admire in the 3D/CG used in the movie. There is a Haikyuu movie coming up, and I think that style is what anime fans would have liked to see with Slam Dunk movie.