Two main reasons. First, Buddhists picture him that way in statues because that is how they want to remember and see him.
Second, and more importantly, this posture fits the central teaching and goal of Buddhism, which is to teach us to release our desires and be at peace with what is rather than what we want life to be. It is about equanimity. One major goal of Buddhism is to be happy despite whatever you are experiencing in this world, proving that you live above it. Not a bad philosophy.
Interestingly, Jesus is often pictured as suffering on the cross. Why this difference? Again, because of the central teaching of the faith and its teacher, Jesus. He said that he came to pay the ransom (penalty) for our sins. His death and resurrection are the central act from his life, and in fact, that is why he came (according to his own words) - to die in our place. This death shows both the horror of sin and the love of God and Jesus, who gave so much to fix the problem.
Not that I mean to preach - in fact, this contrast may cause people to seek out Buddhism, and avoid Xianity. But the contrast is interesting, and makes both faiths worth looking into, imho.
