I remember reading about that at the time, and they said it was because some of the new actors' voices didn't sound right with the reverb. The same article said that they were going to put in a throwaway reference to new surgery for the Visitors' throats, but had decided not to.
As I said, the show-runners were not terribly concerned with logic or continuity. But since most of them had a soap-opera background, you kinda have to expect that....
There was a funny sketch about that on MAD TV several years ago - everybody wanted to talk to him about Willy and nobody believed that he was Freddy - I think he started killing everybody who called him Willy.