Arnor- Designed For Failure?

[Inziladun]

There are stark differences apparent between the Númenórean Kingdoms in Exile in Middle-earth, Arnor and Gondor.

One would think Arnor had the greater importance to the Númenóreans, since that was the realm of Elendil himself, and after the war of the Last Alliance, was to be ruled by his elder son, Isildur. Yet, it fell into decay, conquered by the Lord of the Nazgûl and his army of evil men. Why was that? The fact of disunity among the Northern Dúnedain, splitting Arnor into smaller realms, was an obvious problem. But it has always seemed to me that the Witch-king's victory over them was easier than it should have been. Their Tower of Amon Sûl was taken and razed. The capital city of Annúminas was deserted, and the other city, Fornost, was captured. The reader is told comparatively little about the Arnorian cities and fortifications, but they seem to have been less strong than their southern counterparts.
Arnor also lacked a port of its own, whereas Gondor was a sea-power.

Orthanc was made of an apparently unbreakable rocklike substance and had never been captured. The walls of Minas Anor were fashioned of the same stuff.
Why did the Southern Kingdom have those advantages the Northern Realm lacked, especially considering Arnor was to have been the seat of Elendil?