Northern Line vs. Southern Line

[A_Brandybuck]

While I was reading the Appendix of 'Lord of the Rings', I got stucked at the following text passage in Appendix A:

It was the pride and wonder of the Northern Line [of the Dúnedain] that, though their power departed and their people dwindled, through all the many generations the succession was unbroken from father to son. Also, though the length of lives of the Dúnedain grew ever less in Middle-Earth, after the ending of their kings the waning was swifter in Gondor; and many of the chieftains of the North still lived to twice the age of Men, and far beyond the days of even the oldest amongst us.

What exactly could be the reason for the swifter waning of the Dúnedain of Gondor?
Does the reason only lie in the mingeling with other folk without númenórian ancestry? Was there no or less mingeling in Eriador? In Eriador there has also lived other non-númenórian folk, which could have mingeled with the Dúnedain.

Or does the reason lie in the different behavior of both lines? In later times it seems to me, that the Rangers of Eriador, have kept more true to their ideals than the Dúnedain of the South, although or just hence there was no more great realm in the North. The Dúnedain of the South acted more in things of power and glory, while the Dúnedain of the North acted more unselfishly. In example, by protecting the borders of the Shire. Maybe the presence of Elves or the contact to them let them treat their ideals more noble, than the southern Dúnedain, who had not really much contact to the Elves.

I am interested, which meaning you have on this topic.