DoubeOhFive, like I've said a million times, Kenneth Biller said the character of Kes had a lot of potential for future stories. Not to mention the fact that Kes was featured quite a lot in season 3. So clearly not all the writers were drawing blanks on what to do with Kes.
I still firmly believe that it all came down to who was EASIER to write for - characters like Tuvok and Chakotay who were pretty much the standard character types we'd seen before and could easily get their fill of lines throughout each season by spouting technobabble or in the case of Chakotay hardly anything at all. Only in that sense was Kes "hard" to write for for some writers, since she couldn't just get her fill of lines in any given episode by updating the captain on the shields and actually had to be inserted somehow.
Kes was boring after a while. They did everything they could with her. Apart from actually aging her out and making *that* a story, there wasn't much left that warranted keeping her on another season.
Also that's just your opinion. Clearly Biller, an actual writer on the show, disagreed. And I'm pretty sure I read somewhere Menosky felt there was a lot more juice left in the Kes character too.