In the manga industry, mainstream publications are usually split by target demographic: shonen for boys/young men, shojo for girls/young women (think Sailor Moon, Rose of Versailles, etc), seinen for young men, and josei for young women are the main four, with shonen being THE thing people think of when they hear the word "manga". Before SBR, Jojo was published in Shueisha's Weekly Shonen Jump, alongside such titans as One Piece, Dragon Ball, and Naruto. And until SO, Jojo pretty well fit in with the shonen demographic, bizarre though it is. Fundamentally, it comes down to cool guys punching each other in cool ways, which shonen readers are absolutely down with.
But then SO comes along. Imagine Araki going to his editors at WSJ and saying "yes, this next part will be centered around a nineteen-year-old American woman in prison". It's a stark contrast to the historically-focused Jonathan or Joseph, or the Japanese cool guys Jotaro and Josuke. The whole premise is absolutely orthogonal to WSJ's target demographic; looking at SO as a whole, it's clear that Araki was pushing boundaries - for example, making Anasui first appear as a woman (he "wanted to make a character that transcended gender", which ironically FF ended up doing in the anime).