Tyson was the worst possible fighter for Frank. Short and fast taking Bruno jab out of the picture . Bruno had an excellent jab his main weapon. Without that he was in trouble.
Like said, Tyson knew his style n I think his success in the first gets overstated. I'll argue its Tysons last great performance too but again, he beat him b4 so its not a surprising win.
Imo, a combination of technical & psychological reasons: 1) Technical - Bruno had big long arms. He had a decent textbook/conventional side-on stance, which, allied with his power, meant he had an effective jab and was a decent long range boxer. He was heavily muscled, stiff & slow, which, allied with his long arms, meant he was a sitting duck once an explosive, short armed fighter like Tyson got within their own punching range. He was always likely to be more competitive vs long range, jab & move boxers with long arms, like Lennox Lewis, than against explosive and fast short armed, mid-range fighters like Tyson. 2) Psychological - As other posters have alluded to, in his autobiography Bruno said he knew his eye problems were such that he shouldn't have been cleared to fight. He claimed that George Francis distracted the person examining his eye during the medical by talking to him & that Bruno knew that he wouldn't see punches from certain angles & that it was almost certainly his last fight. i.e. it was a cash out. That mindset, against a fighter Bruno knew from the first fight was his stylistic kryptonite, likely meant big Frank had lost mentally before the first bell.
I think it came down to lack of confidence. His eye troubles, and remembering the tornado he faced in the first fight.
Realistically, seeing how the first went, his only hope was that Tyson had eroded badly and turned up in poor shape. Quite to the contrary Tyson looked as fast and sharp as always, so Bruno must have understood very quickly that he stood no chance.