When Arthur founds the Round Table, he requires all of his knights to take an oath to not commit crimes, to show mercy to those who ask, to give special succor to ladies, and in general to be honorable knights. What's this all about? Chivalry, that's what. The chivalric code, which these knights live by, is all about being honorable, and it's Arthur – the rule-maker – who decides what that means. In this and in many other ways, Arthur is the source of rule and order in Le Morte D'Arthur.
Of course this is not to say that Arthur's legal system doesn't have its problems. For one thing, trial-by-combat, might makes right, regardless of the facts. Could it be that this makes Arthur a little uncomfortable? How else can you explain the fact that he requires Launcelot and Gwenyvere to be caught in the act, instead of automatically challenging him to combat? Or maybe he's and his country are starting to stray from a medieval legal system toward one in which proof is more important than power. Whatever the case, he's throwing into question the system upon which he founded the Round Table at the moment it's crashing down around his ears.
The favor-for-favor or "boon" system upon which feudal relationships rely depends upon the honorable behavior of all members to be sustainable. But people aren't honorable at all, which is why feudalism isn't around anymore.
Arthur totally gets that the trial-by-combat system has some problems, but by calling it into question, he calls into question the very foundations on which the Round Table was built. No wonder it falls apart.