Ethos

With the Magna Carta, it's all about who you know. It gets its power from who agreed to it (and who stamped it at the bottom).

The opening lines list a bunch of rich and powerful English celebrities to give the charter credibility and then make only very basic moral arguments throughout.

Imagine that you're an average medieval free man/woman just doing your thing on your farm and you find out that literally every rich guy you've ever heard of all agreed on this one document. You'd probably be so amazed and astonished that you'd figure that the doc must be super important…and you'd better get on board too.

The document doesn't attempt to logically work through the reasons behind any of the limits it places on the king or the rights it grants to the people. And it's not like the average person could read or had well-developed analytical skills anyway. The only explanations infrequently given are that it's the way things have traditionally been done, or it's the way some important person wants it to be done, which are basically all anyone in the medieval era cared about anyway.

Traditions and obeying rich people was kind of all they had back then.

There are no direct references to anything analytical: no one analyzed the costs and benefits of jury trials or studied the ramifications of a council of barons. While it might have been fun to hold a focus group of knights, monks, and livestock herders, and find out the opinions of the masses on their preferred government styles, there wasn't really time. The Magna Carta was supposed to prevent a war. They couldn't get all emotional.

The Magna Carta is a legal document devoid of any personal stories. By reading it you can't tell who had their lands snatched up by the King, who the prisoners are who're being released, or even what exactly they did to be arrested in the first place.