Here's why:
* Star Trek: The Next Generation: Picard and the crew of the USS Enterprise-D encountered the Borg in the two-part episode "Q Who?" which aired in 1990.
* Star Trek: Voyager: Captain Kathryn Janeway and the crew of the USS Voyager encountered the Borg in the episode "Caretaker," which aired in 1995.
While Janeway encountered the Borg later in the Star Trek timeline, she did spend seven years in the Delta Quadrant fighting against the Borg and their influence.