This forum might help explain things,
Elo is a bit different than the other formats in terms of what people consider polite play. Most players are trying to get their scores up in elo in a time-efficient manner, so if they feel like they have already lost, or will most likely lose except by a huge mistake by their opponent, it's not at all uncommon for them to leave the match and move on. I'm sure you still get some "rage-quitters", but I think for the most part it is just players trying to play quickly.
Try not to think of it as them quitting so much as them conceding the match. If you're like me, I always hope they'll quit the match just because I am capable of pulling out those bone head, lose a match that was in the bag, kinda plays.