had been sitting, was sitting

[joham]

---What's wrong with your coat?
---Just now when I wanted to get off the bus, the man next to me _____ on it.
This was one of China's College Entrance Examination questions. The given answer was 'had been sitting'. I wondered why another choice, 'was sitting', couldn't be used.
Could I ask native English teachers to help me? Thank you very much.

 

[Tdol]

For me:
was sitting - still sitting at the time I wanted to leave
had been sitting - was no longer sitting