I know why people refuse rematches. I refuse and accept rematches with various reasons. Here are some:
1. They don't like to play with you.
2. They don't like your style of play.
3. They think you're too easy or too tough.
4. They want to play with more people to broaden their game knowledge.
5. They just don't have time.
Think about it. Online chess is like sex. Sometimes it's just a hookup with a stranger and you will never meet again (no rematch). Sometimes you feel it's interesting and you two come back for more (rematches, and maybe add friend).
If you believe you'll win, beat them in the first game. Then move on.