This is what I understand about the Land Without Magic. Thousands of years ago, the LWM once had magic that was supplied by Tree Nymphs. And human civilization was more evolved than it should have been thousands of years ago. Isla, a human that despised magic, managed to get in to the grove where the nymphs lived. She and her friends killed the nymphs and destroyed the grove with fire and axes. This results in the land loosing magic.
The only surviving nymph, Gothel, gets her revenge by committing genocide on the entire human race. She kills every human except one, Seraphina. A human who possess magic. Gothel convinces Seraphina to join her. And they travel to a new realm where they can cultivate magic. Because Gothel killed every human, humanity has to start all over. They have to evolve again this time in a land with out magic.