Wiktionary claims that the etymology of the accusative first person plural pronoun ci is < L. ecce hic "look at this here".
This is uncommon in the Romance languages, which have largely retained reflexes of L. nos.
Granted that sometimes things happen in language change without an obvious cause, was there any reason in Italian why this pronoun alone did not descend analogically like vi < L. vos did? (I know loro did not either, but it does descend from L. illorum so it's still not a novel form unlike ci.)