The name Raiden is actually that of a deity known as the Thunder God. It is rumored he received a personal invitation by Shang Tsung himself and took the form of a human to compete in the Tournament.
Raiden's victory comes as no surprise to him. He was never impressed by Shang Tsung's inferior sorcery, Goro's brute force, or the challenge of the other contestants. He quickly becomes bored with his mortal competition and soon invites other gods to participate in the contest. The ensuing battles rage on for years. And the wars result in our world's final destruction. Have a nice Day.