C.E. Evans wrote:
Both the producers and the studio felt that the character wasn't working. When the character of Seven of Nine was conceived, there was already a sentiment by the studio that there were too many characters on the show as it was (i.e., they needed to trim the budget) and so someone had to go.
In other words, Jennifer Lien didn't feature enough boob and wasn't ready to wear skintight catsuits.