Why does Grecani language not exist in Sicily (Magna Grecia)?

We know from Salvatore Cusa’s collection of church deeds from Sicily that Greek remained in use in official contexts until at least the 1300s—with the “correctness” of the Greek gradually degrading.

We know that the use of Greek in Calabria and Salento steadily declined, with much wider areas using Greek in the 16th century.

If the use of Greek was in gradual retreat over the past millennium throughout Southern Italy and Sicily, following the Norman conquest, then… well, then it happens to have retreated quicker in Sicily than in Southern Italy. The Calabrian enclave is certainly relatively inaccessible (that’s why it’s in the ‘Ndrangheta’s heartland).