Cell absorbed 17 and 18 and became perfect and instead of killing everyone. He decided why not test the power of his perfect form so he held a event called the Cell Games and it will be starting in nine days so Earth has to bring it's strongest fighters or else the Earth will be destroyed plan and simple. So when the day came Goku, Gohan, Vegeta, Trunks, Piccolo, Yamcha, Android 16, Krillin, Tien and you showed up but Cell never saw you...are you a new fighter? After he smacked Hercule into a mountain and his fight with Goku...he instead wanted to see you fight next.

Cell: "For the next fight. I choose you to fight me. I have no data or ever seen you before so come up here and fight me. Make this fun too while your at it."