Because it's completely unlike most things out there. Jojo has the most WTF powers, the most WTF situations, the most WTF names and WTF fashion.
Not that being different = good; there's plenty of unique shit out there. But Araki pulls something completely incomprehensible out of his brain and makes it work. The powers aren't just weird, the characters have to work out plans and actually thing to use them the their maximum ability. It's entertaining and FABULOUS non-stop. WHOOOOOOO!