I don't know if he lost it so much as abandoned it. Everything changed, his songwriting went inside, so I guess his singing changed along with that self-reflection or whatever he was doing. He always blamed Twist and Shout for ruining his voice, but I think he just didn't want to do that anymore. They always wanted to do something new and not rehash, so who knows.he lost that range and tone i guess before the beatles were even done.
I always saw that as the real dividing line between Lennon and McCartney - that McCartney never went for introspection the way Lennon (and Harrison) did. That's a generalization, but Lennon would have never sang Listen to What the Man Said or Silly Love Songs or any of the cloying, tra la la lightweight commercial pop that McCartney churned out, and McCartney would have never sang...well, most of Lennon's post-Beatles songs. Just pick one at random. Ha.
But of course it was that dichotomy that made The Beatles in the first place. And it was why both of them were only occasionally great separately, while they were pretty consistently great when they worked together. But then very few songwriters are consistently great throughout their careers. Most peak when they're young.