I suspect it was his knee. Watch some videos on the peek-a-boo style that Tyson employs. You HAVE to be able to drop low very fast and use very quick footwork to close the gap and then use power punches. It is an incredibly hard style to fight against because they’re like a rabbit, bouncing around and slamming you nonstop, chasing you around the ring where you can’t get a moment to think.
Without that, all you’d have to do is keep jabbing and keeping your distance, like Paul did. I suspect they had a no-knockout clause in Tyson’s contract and with his knee hurting, he couldn’t get in close, and Paul played it safe. Didn’t want to be known as the man who beat up an aging legend showing his age.
See this video (jump to around 1:19 for a small demo of what I mean): https://youtu.be/TmJJK7Ac4Fk?si=gu-aJ-pLSSglQ8WV