Fritz Kern explains this in the second and final section of his book, "Kingship and Law in the Middle Ages."  The section is entitled "Law and Constitution in the Middle Ages." Here I will review his comments regarding the mediaeval law.

He begins by contrasting this historic law with what is defined as law today:

For us law needs only one attribute in order to give it validity; it must, directly or indirectly, be sanctioned by the State.  But in the Middle Ages, different attributes altogether were essential; mediaeval law must be "old" law and must be "good" law….If law were not old and good law, it was not law at all, even though it were formally enacted by the State.

Consider how pathetic our society would seem to someone coming from this past time that Kern describes.