Cause you can't proof that this was cheating. I often had "Opponent quit game", "Connection lost" or something like that, while my opponent was leading. Why should someone quit when he is leading? There are many connection related problems which neither you or your opponent can't do anything about it. Especially if you play against users from overseas.
Besides that did crew ask several times not to abuse people for cheating. If you wan't to contribute something against cheaters, read this thread:
General Discussion - Help us find WAYS to cheat!! by powdersnow