Why is a Catalan dialect still spoken in Alghero?

A Catalan dialect is still spoken in Alghero because the town became a Catalan colony in 1354 under Peter IV of Aragon.3

During the period when the Kingdom of Aragon ruled Sardinia—from the 14th to the 17th century—Catalan was used extensively for official purposes across the island. While other regions eventually shifted toward Spanish and later Italian, the Catalan language was introduced to Alghero in the 14th century and has persisted there as a distinct eastern dialect. Today, Alghero remains the only town in Italy where Catalan is still spoken.123