The northern route looks today like the harder alternative taken by people avoiding crowds. Historically the logic was the reverse, and the reason is the frontier.
In the ninth and tenth centuries the interior of the peninsula was contested ground. The Christian kingdoms held the north, the emirate and then the caliphate of Cordoba held the centre and south, and the plateau between them was raided from both directions. Almanzor sacked Santiago itself in 997 and carried off the cathedral bells to Cordoba. A pilgrim crossing the meseta in that period was crossing a war zone. The strip along the Cantabrian coast, protected by the mountains and held continuously by Asturias and then by Leon and Castile, was the route that could be walked, and the monasteries and hospitals along it grew accordingly.
That changed as the frontier moved south. By the eleventh century the interior road was securable, the kings developed it deliberately with towns and bridges, and the Frances became the main artery while the coast reverted to a local road serving its own shrines.
The northern route retains what that history built: the pre Romanesque churches of the Asturian kingdom around Oviedo, which are a distinct architectural school with no close parallel elsewhere in Europe, and a series of monastic foundations along the coast.
UNESCO recognised the position in 2015 by extending the World Heritage inscription of the Camino Frances to include the routes of northern Spain, which covers the Norte, the Primitivo, the Interior Basque route and the Liebana road. The extension was itself an argument won: the coast had been treated as secondary since the twelfth century.