The reasons why this character quit Little Stone are as follows:

1. Personal development: As a young singer, Han Tiantian has great development potential in the music field. He feels that leaving the Little Stone Band will give him more opportunities to expand his personal career and focus on his own music development.

2. Contract expiration: Artist contracts generally have a long term. After Han Tiantian's contract with the Little Stone Band expired, she chose not to renew the contract and instead looked for other development opportunities.

3. Shift of work focus: During Han Tiantian's time in the Little Stone Band, she mainly focused on her singing career. However, during the development of his personal career, he realized that he also had development potential in other fields, so he decided to quit the Little Stones and shift his focus to other areas.