Complex sentence: The reason why the Romans decided to name the entire province Callaecia/Gallaecia, the "land of the Callaeci", remains unknown, since those were just one specific tribe settled in a specific location.
Split into simpler sentences: The reason why the Romans decided to name the entire province Callaecia/Gallaecia, the "land of the Callaeci", remains unknown. After all the Callaeci were just one specific tribe settled in a specific location.