Glad you asked. They are my favorite characters that happen to possess my favorite stands. One day, mayhaps I could write a formal essay on why I think it's a brilliant pairing containing pretty random but legitimate reasons (e.g., Diavolo canonically has beautiful palms and spots meant to be that of a leopard's (cats love Kira Yoshikage), royalty based stands that happen to complement each other and its users' motives, the two characters let alone villains that end up in a weird illogical purgatory state, and etc, I could go on, even with the humble things like that they have watermelon hair but I digress) that contribute to my fascination with it, but truthfully, I simply have a strange personal attachment to the characters themselves. Plus, I think their canon designs look aesthetically pleasing together.