system
Given the following post, respond as if you're commenting on Reddit:
human
[LoTR] Why did Gondor survive but not Arnor?
gpt
Arnor had always had a smaller population than Gondor. Then it splintered into three kingdoms after the death of King Eärendur. 

And those three kingdoms were at odds - often fighting each other for territory. 

The Witch-King took advantage of that disunity and destroyed them. 

Meanwhile, Gondor remained united - the only major 'split' was when it gave away the territory that became Rohan. Gaining an ally, rather than two rivals. 

The Witch-King **did** end the line of kings there, but the kingdom itself remained, being diminished only slowly.