lord of the rings - Why did it take Gondor and Rivendell so long to send aid to Arnor?

Before Eärnil ll took the throne in T.A. 1945 "Gondor came near to destruction." [Gondor and the Heirs of Anárion] The Wainriders sat on their haunches after defeating the Northern Army. Earnil who was a cousin to the King and led the Southern army came up from the South after victory there and took the Wainriders out. It was only 4 years after Arnor and Gondor started to communicate again that King Ondoher died [1944] in this war along with his two sons. It is said that the reason that the North and South kingdoms made contact [1940] with one another again was because

they perceived that some single power and will was directing the
assault from many quarters upon the survivors of Numenor. [Gondor and
the Heirs of Anárion]

Earnil was never quite secure in sending men to aid Arthedain for quite some time after becoming king [around 30 years] and when he finally did send help after receiving dire messages from the North he could only send a a small force. However, even at this point in Gondor's waining they have a clear advantage over the North in strength.

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.... when
Eärnur came to the Grey Havens there was joy and great wonder among
both Elves and Men. So great in draught and so many were his ships
that they could scarcely find harbourage, though both the Harlond and
the Forlond were also filled; and from them descended an army of
power, with munition and provision for a great war of kings. Or so it
seemed to the people of the North, though this was but a SMALL
sending-force of the whole might of Gondor. [Appendix A., Gondor and
the heirs of Anárion]

As to the Elves, Arthedain in particular remained friendly with them and were receiving help from Lindon and Rivendell for their long war with Angmar.

Amlaith and his descendants maintained friendship with the Eldar,
especially with Cirdan at the Havens. [The Heirs of Elendil, Chapter
7]

When the other two kingdoms are ravaged Arthedain holds out with "aid from Lindon and Imladris". Before Angmar invaded Arnor in 1409 Rivendell had been beseiged, and during that invasion Weathertop was destroyed. The last Prince of Cardolan [such as the one in Dol Amroth] was killed when this invasion came. The Dúnedain held out at Fornost with aid from Lindon.

Araphor son of Arveleg was not yet full-grown, but he was valiant, and
with aid from Círdan he repelled the enemy from Fornost and the North
Downs. [Appendix A: The North-kingdom and the Dúnedain]

For the most part the Dúnedain held out at Fornost, the North Downs, and at Tyrn Gorthad [those fighting here were without their Prince who had died but was said to have been buried here, "some say the mound in which the Ringbearer was imprisoned had been the grave of the last Prince of Cardolan" - The North-kingdom and the Dúnedain].

So the North Kindom did get help from Lindon and Rivendell. Elrond even sent for Elves to help from Lórien, but clearly it did not do much good. For a lot of this time the 3 seperate Kingdoms in Arnor were fighting Angmar. For about 300 years before Arthedain fell it was really Arthedain versus Angmar. The Dúnedain in Cardolan were wiped out finally in the plague in 1636 and Rhudaur had long fallen before the Plague to Angmar.

Basically it was slow decay and constant picking away at both kingdoms in decline. Both kingdoms aided in their decline with in-fighting, the split in the North, the Kin-strife in the South and a further diminishment of the royal line in the South with members of that line renouncing their inheritance and joining rebels since they were fearful of the jealous kings.