Why would a girl reject a nice guy if she's single?

1. She might not be prepared for it. LOve/marriage etc may not be in her mnd.
2. Parents might have taught her to reject all love proposals and marry according to their wish.
3. Being a nice guy doesn’t mean girl will fall for you. She has her choice. We don’t know what she is looking for in her future husband. Time will prove.
4. May be she don’t want a love marriage. Her concentration be on career/studies.
5. You have not mentioned if you both are from same community. Social status/religion/culture etc. This might also stand as block.

If you are gentle man/ nice guy move ahead without disturbing her. You will get a life partner which suits you.