Serena fought bravely by herself so that her friends would not have to get involved again. But the evil and strong Cardian was almost too much for her. Raye, Amy, Lita and Mina regained their memories and transformed back into Sailor Scouts to help save Sailor Moon™.
It is the destiny of the Sailor Scouts to guard Sailor Moon.
From Episode #148 “So You Want To Be In Pictures”