Why is Breton considered an Insular Celtic language rather than a Continental Celtic language?

Breton is classified as an Insular Celtic language rather than a Continental Celtic language because it is not a native dialect of the European mainland. Instead, it is an importation brought to the Armorican peninsula (modern-day Brittany, France) by migrants from southwestern Britain during the 5th and 6th centuries CE.12

Linguistically, Breton is too similar to Cornish—an Insular Celtic tongue—to suggest any significant influence from the indigenous Continental Celtic language, Gaulish, that may have existed in the region at the time of the migration. Like other Insular Celtic languages, such as Welsh, Irish, and Scottish Gaelic, Breton shares specific grammatical and phonological characteristics, including the system of initial consonant mutations that developed within the British Isles.123