Because I am older.

According to People's Daily Online, the reason why Oshima Yuko graduated is because of her old age.

When Oshima Yuko participated in a music variety show on Japanese TV, she was asked, "When did you start thinking about graduation?" Oshima said that he suddenly realized this problem when he was 19 years old. At that time, he thought about graduating when he was 22 or 23 years old. As for the reasons for graduation, Oshima said that on the one hand it was due to physical strength, and at the same time he also wanted to do something other than AKB48 (such as acting).