Lennon was though by definition, a throatier singer than Macca.
Do you mean he sang more from the throat than the diaphram?
If that's true(I can't really tell myself) then I have heard that puts a lot more strain on the vocal chords which definitely could explain it. Singing from the diaphram takes a lot of the pressure off apparently.
And I guess some of it could just be in the "genes", it's possible Macca just had generally stronger vocal chords. Sure he can sound rough now, can't sustain the notes as long, because I don't think he really took great care of his voice either for much of his career, but it's possible that besides maybe naturally developing a singing style less from the throat, he just might have had chords more able to stand the wear and tear over the long term.