According to Wikipedia:
"Before Trebek's second season, contestants were able to ring in at any time after the clue had been revealed, and a buzzer would sound whenever someone rang in. According to Trebek, the buzzer sound was "distracting to the viewers" and sometimes presented problems, as contestants would inadvertently ring in too soon, or ring in so quickly that by the time he finished reading the clue, the contestant's five-second limit had expired. He also said that, by not allowing anyone to ring in until the clue was finished, home viewers could play along more easily, and faster contestants would be less likely to dominate the game."
