3. Successful Fighters Were Considered Chosen By God
In a highly religious era, the victor in trial combat was believed to have been chosen by God in his righteousness. It also very conveniently protected the referees and winner of a trial from any accusations of cheating, wrongdoing, etc. since that would be tantamount to questioning the judgment of God himself. In practical terms, the men of the era believed the god would not let an innocent and virtuous man die, so it would be impossible for the wrong man to be killed in a trial by combat.
Of course, in practice, this is incredibly far from the truth, and likely many medieval men died who were genuinely innocent due to lack of martial skill, bad luck, cheating, or any combination of the above. It was these arguments of the lack of real justice that ultimately led to the practice being outlawed throughout Europe in the 17th through 19th centuries. It is also likely why the method never caught on in my areas, with the practice being unheard of in Anglo-Saxon law before the Norman Conquest, in Islamic law and throughout much of the rest of the world. Even many medieval lawmakers felt that stabbing or bludgeoning your fellow man to death wasn’t a great way to prove your worth as a person.