It's easy enough for a foreigner to learn it in a year (so all what's left is to expand the vocabulary and polish the pronunciation). How can it be hard enough

It's easy enough for a foreigner to learn it in a year (so all what's left is to expand the vocabulary and polish the pronunciation). How can it be hard enough to prevent natives from learning "proper" written French? Was it also illegal to teach them so being hard enough to be impossible to figure out intuitively was sufficient?