The history behind the design
People Over Power sums up the rule of law that began with Magna Carta 1215: even the king answers to the law.
The most lasting idea in Magna Carta 1215 is that the king himself is bound by the law. Clause 39 protected every free man from being punished except by lawful judgement or the law of the land, and Clause 61 set up a council of barons to enforce the charter if the king broke it.
Later generations, from the seventeenth-century parliaments to the framers of modern constitutions, built on that principle that power answers to the law, not the other way round.