Well, naturally things somewhat depend on how you structure your world's social and economic side as well as the mentality of 'typical' mages in your world, but several explenations come to mind:
Just like real school, learnign magic costs money. Look at the cost of a single first level spell, to get a scroll to 'learn' it. Combine this with (generalizing a bit and making some assumptions) the idea that in most medievel societies the difference between the haves and the have nots was pretty darn big, and those who were poor were really dirt poor, needing to work 7 days a week, 12 hours a day all year long just to get by. Learning magic also costs a lot of time. Unproductive time.
Furthermore, there is the matter of logistics. How many mages are there? How much time are they willing to spend to teach others?
Also, why should they? It infringes on their 'monopoly' through which they can charge those exorbitantly high prices for their services.
Also, depending upon your world's way of handling magic, in most novels, magic is also scary stuff. If you mess up you die or go mad.
Furthermore, intelligence can be a real issue. Ever look around you and realize how DUMB most people out there are? The vast majority of the NPC's prolly do not have the requisite intelligence to understand even the most basic concepts of magic. Compare it to rocket science, philosphy, religion, nuclear physics, medicine, quantum physics and relativity theory thrown all together to understand all aspects needed to understand the principles, cast and cause the right effect for spells adequately.