I'm guessing there are several reasons: a) not the color they wanted, b) thought twice about playing the opponent they selected, c) decided they didn't really want to play a game after all (e.g., scared, need to go), and d) they have no idea what chess etiquette is, having never played a single game of OTB chess for stakes.
Those are my guesses. I never abort a game that I have agreed to, so I can't add my personal reasons. I have aborted games as Black after waiting a minute or so for the other player to move and seeing no response.
We may never know.
Why do people accept challenge for a Live chess game and then abort ? Whats all that about ?