Despite the fact that the Northern Kingdom held the High-Kingship, it didn't take long for Gondor to become the dominant kingdom of the two. They stayed more powerful for much longer, and I believe Pelendur, in particular, held the kings of Arthedain in some contempt, despite Ondoher's attempt to reconcile via marriage of the Prince Arvedui and Princess Firiel. Tolkien notes that the Line of Stewards, though they swore to hold the rod until the King returned, never expected them to return and didn't want them to. This opinion is exemplified in the attitudes of Boromir (of the Fellowship) and his father Denethor II, especially, toward the return of the King. Notice that Cirion never made mention of seeking out the Chieftain of the Dunedain, to offer him the crown, simply that his father could not claim the crown himself. He did make the point that they already wield the powers of the King, and that if they want to continue  doing so, they should uphold the status quo.  To a ceratain extent, Cirion agrees with his father's evaluation of the Line of Elendil, and they agree with Pelendur's decision to withhold the crown from Arvedui and grant it to Earnil, but in the absence of any heirs of the southern line, Cirion doesn't see a legal way to withhold a legitimate claim by the Heirs of Elendil in the north. Thus, keeping things as they are now serves their purposes without  taking any risk.

To to be fair to Cirion, he isn't really power hungry, he honestly believes the Line of Stewards has done their duty and is the best choice for the rule of Gondor, but he also won't wrongfully withhold te crown from a rightful claimant. He's trying to do what is best for Gondor while still remaining true to the mission of their family: to be caretakers of the kingdom in the stead of the kings.  Boromir, as you can see, has a different perspective. i leave it to you to decide whether he's wrong in his belief that his family deserves the crown.