Why was Lena recycled in the concave and convex world?

It was eliminated so it was recycled.

After Lena was eliminated in the final elimination round, she was recycled according to the rules. In the end, only Jin survived.

"Convex and Concave World" is a 3D online animation produced by Qichuangshe. The pilot episode 0 was broadcast in November 2015, and the first season of the animation was updated on June 28, 2016, with a total of 32 episodes. The second season will be updated at 10:00 am on October 8, 2017, with a total of 20 episodes. The third season will be updated every Friday at 12:00 noon starting from January 25, 2019, with a total of 40 episodes. The fourth season will be updated every Friday at 12:00 noon starting from April 29, 2022.