He was past his psychological and technical peak when he faced Holyfield. Evander Holyfield was a very good heavyweight and an absolute great at Cruiserweight. Tyson was small enough to have had a go at Cruiserweight, if it had been established a few years earlier, and he'd have probably wreaked absolute havoc amongst the division.
When he destroyed the reigning heavyweight division in the Eighties, he was viciously precise when the openings came. he was also patient, which is a quality he lost quite early - maybe coming to believe his own hype as the indestructible beast. By the time of his first fight against Frank Bruno, he was already becoming more of a brawler - only 4 years after his professional debut.
Sometimes the light that burns twice as bright lasts half as long. When d'Amato died, it wasn't long 'til Don King had exclusive access to the golden goose of boxing.