They know you are probably a tough opponent to beat and only fluked the win.
Blitz and Rematch

I offered a rematch to one player and he thought I was insulting him!
The players here are very strange. I almost always offer or accept a request for a rematch.
Here are the main exceptions for me:
A) It's a tournament
B) I have time for only one game
C) My opponent is abusive. This doesn't happen very often.

I accept most rematches, but there are some instances where the quality of the game was so bad that I couldn't accept a rematch in good conscience. If the other player can fight back so that there is the element of danger of losing, I'll accept in a heartbeat--after all, playing competitive chess is what online chess is all about. But if he can't make it to move 10 without making major concessions, I won't accept. There's no fun for me in that kind of winning.
So why is it if you try and rematch anyone in blitz they almost always ignore you and or decline. In multiple games I have had the upper hand and lost on time by a second or so and attempt to rematch and it is like listening to crickets chirping.