2 reasons :
1. physical looks (face, body and boobs) for example : Maya Kawamura, Yui Oba, Hibiki Otsuki, Wakaba Ono, Ai Uehara, Honoka Orihara, Maria Ozawa, Natsume Yuki, Nana Aoyama, Rio Hamasaki, Satomi Suzuki and many others
2. performance, may not like their face & body but if they have agressive performance I may like them, for example : Uta Kohaku, Noa, Reiko Kobayakawa, ecc