Cause you didn't really beat the guy did you? It depends on how many moves have been made and what sort of advantage you enjoyed when the game was called.
My rating did not change even though I've won !
I've experienced a similar thing before. Apparently if your opponent times out within a few moves (as was the case), then neither player's rating is adjusted as you had barely done anything to beat him.
That's my theory.
I'm going to assume you're referring to your recent online game against user "tam-thien" which ended after his time expired on the 2nd move.
I think that there is a minimum move count for a game to affect ratings- it might be 4, but I'm not sure anymore.
It's a good rule. Otherwise, you might have received a new rating which would have inaccurately reflected your up-to-date performance on this web site... I don't think anybody wants that!
How is it that my rating did not change even though I've won on time ? My rating was 1582 and his 1438 or something like that.