because i was tought this thing in elementary school during the PE lessons, and some people, it seems, were not:
By disrespecting your oponent, even beaten one, you strip your victory of any actual value, you make it worthless. By congratulating the winner as well as respecting the loser you do not only give respect to your oponent, you respect YOURSELF.
This is like the ABC of sportsmanship.
(dunno why, just wanted to remind people about it)