A big part of Voyager's problem was that the writers had absolutely no clue what to do with Kes. You had one of the main characters being just kind of there and largely useless. Once they brought Seven of Nine in and dropped Kes Voyager got a lot better. The writers had a clear understanding of how to write for Seven and she had a ton of character development over the final four seasons. Hell, I'd even say they did a better job with Neelix once they got rid of Kes, since they couldn't keep falling back on "Neelix is super jealous of Kes interacting with any male on the crew" BS they did a lot of. (And dear god was that annoying as hell. And I like Voyager!)