Over the ~3000 years that Gondor existed, the Numenorean strain gradually became diluted with that of "lesser" (read: ordinary) men. The same happened in Arnor, but the royal line in the North was less diluted that that in the South. That's why Aragorn lived longer than 200 years; extended life was one of the gifts given to the Numenoreans. Even long after they were exiled, members of the royal line lived to three times the span of ordinary men.
I had always attributed Aragorn's longevity to his Elvish and Maiar blood.