Like any other document, control gives power.
Those who hold the power aren't likely to share it.
And since they control the document, they can disseminate what it contains with their spin of things. Keep the peasants ignorant and even if they get ahold of the document they'll never be able to tell that you did them wrong.

One of the big problems with the stone tablets given Moses was that they were carved away from the mountain. The world would probably be a little better off had they been written out in the stars for all to see as "proof" and no one could tamper with the wording.