Why didn't Gondor respond more quickly?

They were stretched thin:

Eärnil was a wise man, and not arrogant, even if, as to most men in Gondor, the realm in Arthedain seemed a small thing, for all the lineage of its lords.

 

He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and the needs of the South-kingdom, "but I do not forget the loyalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil should be estranged. I will send to your aid when you have need, so far as I am able."

 

It was, however, long before Eärnil felt himself sufficiently secure to do as he promised.

 

^{_{Return of the King Appendix A "Annals of the Kings and Rulers" I "The Númenórean Kings" (iv) Gondor and the Heirs of Anárion}}

Gondor is just coming off of a bad century, with the Wainrider invasion right on the heels of a massive plague. Eärnil made his promise in T.A. 1945, shortly after he finally defeated the Wainriders, and Gondor wasn't ready to commit to a major operation.

Why didn't the Elves respond?

This isn't clear from the text. It's entirely possible that the Elves didn't 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}}

But we can't be sure.

Why didn't Gondor respond more quickly?

They were stretched thin:

Eärnil was a wise man, and not arrogant, even if, as to most men in Gondor, the realm in Arthedain seemed a small thing, for all the lineage of its lords.

He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and the needs of the South-kingdom, "but I do not forget the loyalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil should be estranged. I will send to your aid when you have need, so far as I am able."

It was, however, long before Eärnil felt himself sufficiently secure to do as he promised.

^{_{Return of the King Appendix A "Annals of the Kings and Rulers" I "The Númenórean Kings" (iv) Gondor and the Heirs of Anárion}}

Gondor is just coming off of a bad century, with the Wainrider invasion right on the heels of a massive plague. Eärnil made his promise in T.A. 1945, shortly after he finally defeated the Wainriders, and Gondor wasn't ready to commit to a major operation.

Why didn't the Elves respond?

This isn't clear from the text. It's entirely possible that the Elves didn't 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}}

But we can't be sure.

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}}

Why didn't Gondor respond more quickly?

They were stretched thin:

Eärnil was a wise man, and not arrogant, even if, as to most men in Gondor, the realm in Arthedain seemed a small thing, for all the lineage of its lords.

He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and the needs of the South-kingdom, "but I do not forget the loyalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil should be estranged. I will send to your aid when you have need, so far as I am able."

It was, however, long before Eärnil felt himself sufficiently secure to do as he promised.

^{_{Return of the King Appendix A "Annals of the Kings and Rulers" I "The Númenórean Kings" (iv) Gondor and the Heirs of Anárion}}

Gondor is just coming off of a bad century, with the Wainrider invasion right on the heels of a massive plague. Eärnil made his promise in T.A. 1945, shortly after he finally defeated the Wainriders, and Gondor wasn't ready to commit to a major operation.

Why didn't the Elves respond?

This isn't clear from the text. It's entirely possible that the Elves didn't 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}}

But we can't be sure.