Seeing that fate is such an important force in Jojo's, fate most likely chose Giorno over Diavolo due to Giorno's righteous behavior. King Crimson essentially allowed Diavolo to avoid Fate's decisions, so Fate would choose Giorno to be the one to stop him. Just my head canon, but Fate is still an important concept in Jojo.

@Dniuhfdsehefeu Why would you make "the arrow meant for giorno by fate itself" and "and he wasn't worthy" 2 different options when it's just saying the same thing in 2 different ways?

Diavolo was never stabbed by the arrow

Nothing indicates he wasn't worthy

hand slipped