I think it's because the author focuses on Dio. Part 1 introduces Dio, so it's included, Part 2 doesn't have Dio, so it's out, then Part 3 brings him back so it's in.
Likewise, Part 4 is also excluded because Dio is absent. Part 5 also doesn't have Dio, but Dio (and Jonathan's) son is the MC, so you can still continue the (alternate) story this way. (Unlike Part 4, where the story is kicked off by the family members of one of Dio's henchman, making it hard to twist to fit the altered narrative without writing a lot of new things from scratch.)
Since part 6 has Dio in it. It's right in the crosshairs of being next after this part.
It's said that he is strengthening his body with a special breathing technique, and he is said to be the strongest Stand user in the world! How awesome is that!?Everyone! Surely you've heard of "Joestar Family", haven't you? Yes! That celeb-like famous family!That Joseph's grandfather, Jonathan Joestar is still alive and kicking at over 130 years old!That real-estate king Joseph Joestar, for example. He's crazy famous, isn't he?
And... After this point we'll be talking about something like a rumor, or you could say, an urban legend...Jonathan's brother, DIO, is also still alive! Moreover, word has it that he's a vampire!Since he's a vampire, he's immortal, and it's said that he can create zombies alike...! How crazy is that!Anyway, a certain mad scientist came up with an idea...If she could combine the DNA of the two brothers, she could create the strongest life form ever!
