There are currently eight episodes of JoJo's Bizarre Adventure, and a ninth is forthcoming. The first six parts are a family saga told in chronological order, but the seventh and eighth parts, Steel Ball Run and JoJolion, take place in a totally different universe, which can be confusing for newcomers.
In Steel Ball Run, Johnny Joestar, the main character, is modeled after Jonathan Joestar from Phantom Blood, and the spiritual superpowers known as Stands are still in use. Other characters are emulations of classic JoJo characters. Other than that, the Steel Ball Run universe is not canonically related to the first six installments. 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.
Why Did Araki Hirohiko Use Steel Ball Run to Reset the Universe?
First off, Enrico Pucci's Stand, Made in Heaven, which was transformed at the conclusion of the sixth part, Stone Ocean, had already reset the universe. Pucci intended to use his Stand to speed up time until the universe was destroyed and then recreated with each person having a subconscious memory of what happened in the first universe and knowing their own fate as a result.
However, Emporio Alnio defeated him before he could achieve his ideal universe, resulting in the creation of a new timeline. In this universe, Jolyne Cujoh is now known as Irene, all of the characters who died during Stone Ocean are still alive, and no one except Emporio remembers the original timeline. It also appeared that the Joestar family was no longer destined to fight, as no enemies or Stands were mentioned 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".
Is JoJo's Steel Ball Run Considered Canon?
Furthermore, despite the fact that many of its characters are counterparts to characters from the original JoJo universe, Steel Ball Run was not originally published as a JoJo manga. When it first appeared in Weekly Shonen Jump in 2004, it was simply titled Steel Ball Run and was not advertised as part seven of JoJo. The following year, it was relocated to Ultra Jump and confirmed to be an official JoJo part set 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.