Expansion of Magic
The general ability of any one to use magic depends on the amount of magic they believe to exist in the world around them. That in turn depends on how often they see or experience magic, or learn of it, and that in turn depends on how often magic occurs - which in turn depends on peoples' general ability to work magic.
Over the course of history, the world experienced more and more magic. In the beginning, magic only rarely occurred; but as it occurred more often, more people became able to use it. Technologies and ideas allowing for more people to work magic also helped expand this scope of magic. By modern times, the same ways as those in ancient times are far more likely to succeed than in ancient times. Additionally, the invention of a variety of new ways, many of which were much greater in scope, allowed anyone is able to work magic.