It appears to me that the original purpose of the Interdict was as a form of DRM for strong spells. If you want to disseminate a spell, you can't just take a book, magically duplicate the book, and pass it out to all your friends. Learning a powerful spell thus required a willing trainer to teach you the spell, and you couldn't duplicate the knowledge contained in your mind until you knew the spell well enough to teach it yourself (and teaching something is harder than just being able to use it).

If you take this as a starting point, it's just basic classism at play motivating Merlin. All the wrong sorts were getting literacy and learning spells from books, taking away from the right sorts, who had learned from tutors, exclusive clubs, and networking with other powerful wizards. And it probably worked exceedingly well for several hundred years until those damn four decided to start up a school and accept everyone to be taught.