I think magic is like a talent. Anyone in theory can learn to do it, but some people just have a knack for it.
Just like in real life, there are some people who are just better cooks than other people. You can give you people the exact same cook book or cooking teacher, but one in theory can just pick up that skill a bit better than someone else. Some people are just better writers, some people are better at math, and in marvel some people just happen to be good at using magic even if in THEORY anyone can learn it. Along with of course hard the hard work and discipline that goins into it if you really want to be good at it.