WARNING: This article contains spoilers for JoJo’s Bizarre Adventure: Stone Ocean by Hirohiko Araki.
JoJo's Bizarre Adventure currently has eight parts, with a ninth on the way. The first six parts are a family saga told in chronological order, but something that can be confusing for newcomers is that the seventh and eighth parts, Steel Ball Run and JoJolion, take place in a completely separate universe.
Several characters are counterparts to classic JoJo characters, for instance, Steel Ball Run's protagonist Johnny Joestar is based on Phantom Blood's Jonathan Joestar, and the spiritual superpowers known as Stands still exist. But other than that, the universe established in Steel Ball Run has no canonical connection to the original six parts. Although that may seem like an odd choice from writer Araki Hirohiko, there are a few reasons behind the decision to reboot the JoJo universe.
Firstly, the universe had already been reset at the end of the sixth part, Stone Ocean, as a result of Enrico Pucci's transformed Stand, Made in Heaven. Pucci planned to use his Stand to accelerate time until the universe was destroyed and remade as one where every human subconsciously remembered the events of the original universe and therefore knew their own fate.
However, Emporio Alniño managed to defeat him before he could achieve his ideal universe, although a new timeline is still created. In this universe, protagonist Jolyne Cujoh is now known as Irene, all the characters who died during Stone Ocean are alive, and no one but Emporio has any memories of the original timeline. It also appeared that the Joestar family are no longer destined to fight as nothing is mentioned about enemies or Stands in this new world.
So, with the original JoJo universe gone for good and seemingly replaced by one where the Joestars no longer go on adventures, it would be impossible for Araki to continue writing stories in the original continuity. This is why Steel Ball Run takes place in a completely new universe that is separate from even the "Ireneverse".
In addition, it's worth noting that Steel Ball Run was not originally published as a JoJo manga at all, despite many of its characters being counterparts to characters from the original JoJo universe. When it began serialization in Weekly Shonen Jump in 2004, it was simply titled Steel Ball Run and was not advertised as being part seven of JoJo. The next year, it moved to Ultra Jump and was confirmed to be an official JoJo part taking place in a new universe.
Araki has claimed that he always intended Steel Ball Run to be the seventh part of JoJo and that initially leaving JoJo's Bizarre Adventure out of the title was an editorial decision, but it's easy to imagine that Steel Ball Run could have continued to be published as a separate series. After all, Araki has used the JoJo characters for non-canon stories before, such as Jolyne, Fly High With GUCCI, which saw Jolyne interacting with characters who should canonically be dead, and Deadman's Questions, which followed the fourth part's antagonist as a ghost but is otherwise unconnected to the main series.
Therefore, there are two main possibilities for why Steel Ball Run takes place in an alternate universe: because the ending of Stone Ocean reset the universe anyway, and because Araki's editors wanted to be able to market Steel Ball Run as a completely new manga at first. The eighth part of JoJo, JoJolion, is also a part of this new continuity, taking place many years later but showing what happened to Steel Ball Run characters such as Johnny and Lucy Steel after the events of their part. It is safe to assume that Part 9 will also take place in this universe and that JoJo's continuity won't get any more confusing for now.
