Per Wiki:
Voyager’s producers reluctantly fired Lien after her personal issues affected her reliability on set. At the time, this was not openly discussed and her departure was attributed to other reasons; these ranged from Lien voluntarily leaving to pursue other roles to her being removed to make room for Jeri Ryan’s introduction as Seven of Nine.
It’s just too bad Kes wasn’t a very compelling character. Seven, on the other hand, allowed for interesting writing alongside Jeri Ryan’s superior acting prowess.
Agreed.