Why does JoJo start over with a new protagonist in every part?

In creating the generational story, Araki thought about death, particularly the death of his grandfather, and about the legacy people leave behind for their descendants. Two family sagas stood behind the shape: Roots, for its family-centered story, and East of Eden, for its intertwined destiny and rivalry between two families, the pattern Joestar and Brando repeat for a century.

He also knew the risk had been taken successfully before: stories that were well received after changing protagonists influenced his decision to kill off Jonathan Joestar and write a generational story, passing Jonathan's spirit to his descendants. At the time it was unheard of for a main character to die in a Weekly Shonen Jump series. JoJo did it in its first part and never went back.

The handover is not a recast. Araki stated he wanted a different type of main character for every part: Jonathan serious and honest, Joseph a trickster, and so on down the line. The heroes keep a physical resemblance deliberately, so the lineage reads at a glance even as the personality, the era, and the genre change around it. His stated aim for the family focus was a feeling of pride, and of wonder and mystery around the lineage.

That is the whole machine: kill the hero, keep the blood. Each part gets a clean start and a new genre, western, prison story, mafia story, horse race, while the star birthmark and the name tell the reader exactly whose story it still is.