Perhaps because a portion of the audiance had had enough of it. You can watch a show or series of shows happily for hundreds of hours, forgiving minor details but then something happens and a show does something that breaks that suspension of disbelief. From then on in the show has to got to work twice as hard to get that suspension of disbelief back. So perhaps for viewers who came to ST via VOY were more forgiving and who then came later to the earlier Treks might be less forgiving of those. For those that enjoyed VOY as it was great, those that critise VOY in some if not most cases felt it could have been so much more.