the reason is because in a tournament, effective stack sizes are smaller. when your opponent is out of position and he checks to you, he is likely to do the same thing no matter what you bet, so by betting small you lose less when you are bluffing and effective stack sizes are so shallow that when you have the best hand you will be playing for stacks anyway despite a 1/5 pot size bet on a flop. basically its another way to exploit positional advantage.
