A more interesting question is why Portuguese is formed like that, when it should have been פורטוגלית.
It is just an hebraization of either the English or the Portuguese (Portugues). Anyway, radagastthebrown is correct that איטלקי is probably derived from Green or Latin, but it is not recently derived. The word appears in the Talmud.
According to Wikipedia (https://he.wikipedia.org/wiki/%D7%90%D7%99%D7%98%D7%9C%D7%A7%D7%99%D7%9D) it's either from Greek (italikos) or Latin (italicus).