Shang Tsung never chose anyone to compete in the tournament. As host, he only extended the invitations. Chosen warriors from each realm were preordained by the Elder Gods. So how did Shang become host? It was complicated... (thread) https://twitter.com/davesavage1982/status/1357324967189368834

Originally, we referred to the tournament as The Shaolin Tournament. It was hosted by the Shaolin who did the extending of invitations because at that time (hundreds of years ago), Earth was doing the winning with The Great Kung Lao as its champion. (2/10)

Here is where it gets complicated... in MK1 the concept of Outworld was not fully developed, but we knew there was another realm competing in the tournament against the interests of Earth to unbalance the furies and that Shang Tsung was corrupted by its influence. (3/10)

...Shang Tsung is not a native of Outworld. He is from Earth. He competed in the Shaolin Tournament as a representative of Earth and was its Grand Champion for a short time. It was during his reign that he gave in to Outworld influence and switched sides. (4/10)

In my mind, Shang had a complex past. He was born of dual realms, one parent from Earth and the other from Outworld, enabling him to cross realms and ultimately choose his allegiance. This made him a target for Outworld corruption and a precarious representative of Earth. (5/10)

Shang Tsung’s soul stealing power was meant to imply that he literally sold his soul in exchange for the immortality that he enjoys. That power was granted to him by Shao Kahn in exchange for his traitorous action against the Shaolin Tournament hosts and Earth itself. (6/10)

...But, the Shaolin had an answer with The Great Kung Lao who was able to defeat Shang Tsung. Humiliated and now a traitor to his home realm, Shang disappears for many years. Presumably, he goes to Outworld where he plots his revenge. (7/10)

...Shang eventually returns to the tournament but this time he brings Outworld’s Shokan prince Goro to compete. The Great Kung Lao is defeated and Shang Tsung becomes host of the tournament that he once lost as a competitor. (8/10)

...The above was never explicitly detailed in the old games. It was implied in the first couple of pages of the original MK Collectors Edition comic book and the follow-up MKII comic book. (9/10)

...Shang is a great villain, but dig deep and he may have also been a very tragic hero. It’s a compelling story that I wish we had explored further. Maybe they did/will in later games. (10/10)

I should add the above isn’t new info. In fact, the old MK Conquest tv series explored events after The Great Kung Lao’s victory over Shang. In that show, Shang was dragged back to Outworld and imprisoned in the cobalt mines of Kuatan where he plotted revenge. It’s deeeeeep lore.