Kes was a terrible idea for a character that's way too high concept for that show's writers to execute. Her relationship with Neelix was awful, she made Tom look like a creep for and both actors were relieved to see that triangle end. The only thing Kes had going for was that she stood up for the Doctor every time no matter what and they had a good dynamic, but then she left and he had matured enough to stand up for himself and didn't need a surrogate.
On top of that the writers could not balance is cast. DS9 had the core cast, Jake, Nog, Rom, Leeta, Cassidy, Garak, Keiko, Molly, Kai Winn, Dukat, Damar. Voyager? They coudln't think of a way to include 7 of 9 without dropping someone because they thought they had too many people in the cast even after they dropped Samantha Wildman, every crewmember from the Equinox, Lt. Carrey, and eventually Naomi Wildman.