Because it took Arnor that long to ask for assistance.
It's clear reading Appendix A that this was viewed as an internal, Arnorian conflict. It's entirely possible that the Elves didn't even know the war was going so poorly until Eärnur arrived at the Grey Havens:
[A]t last in the autumn of 1973 messages came to Gondor that Arthedain was in great straits, and that the Witch-king was preparing a last stroke against it. Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he could, and with as great strength as he could spare. Too late. Before Eärnur reached the havens of Lindon, the Witch-king had conquered Arthedain and Arvedui had perished.
Return of the King Appendix A "Annals of the Kings and Rulers" (iv) Gondor and the Heirs of Anárion