The first major death blow came in 1988, when Roy Jones, maybe the best boxer to ever compete in the Olympics (even better than Sugar Ray Leonard) was screwed out of his medal. It was such a horrible moment of corruption it started a downhill cycle.
Then you had Jerry Dusenberry, who ran USA Boxing and basically ran out the pro trainers like Lou Duva, Angelo Dundee, Eddie Futch and others who used to basically help train the Olympic guys in their downtime. He basically told him he didn't want his amateurs being soiled by these pro trainers.
Dusenberry was eventually convicted of child molestation around 2000.
Then you had USA Boxing bringing in Manny Steward as their new director of operations in like 2001 and he lasted maybe a year because USA Boxing executives wouldn't listen to him.
So basically 95% the problems are internal. They do a lot of throwing spit against the wall to see that it sticks. Michael King used some former college football players and brought them in because he believed that the best athletes would transition to boxing well. But one year of training against someone with 20 years of training will still get you embarrassed on the Olympic stage.
In reality you can go back even further, the 1984 Olympic team (Mark Breeland, Pernell Whitaker, Holyfield, Biggs, etc.) were being shown on TV as early as 1982 with the Amateur World Championships and then in their exhibitions against USSR, Cuba, Mexico, etc. etc. These were big Saturday Wide World of Sports telecasts and then in the Olympic trials. Mark Breeland was a household name two years before he competed in Los Angeles because he won the World Championships, which was televised.