I had just assumed that the Kes character didn't resonate as the producers had liked, and replaced with Seven of Nine which was a retooling attempt to help sagging ratings.
Personally, I find Seven more exciting than Kes. But the argument for drug abuse makes it rather ridiculous when you consider Kes appeared in one or two flashback-y episodes afterwards. Didn't she? One where she was "evil" and another when the ship was split into different timezones?