They never realize what they're getting themselves into, and are always in over their heads. It becomes too hard, people stop showing interest, or they simply just can't manage to keep it up. Life gets in the way, video games get in the way, or most often interest just fades. They'll get new ideas and start new projects, or reimagine their own project and start over, never getting anywhere. Most fan games are learning experiences, and the farther you get into them the more you realize how poorly made and obsolete the beginning of the game is. Everyone likes to think that they can be the one to make an amazing, ambitious Zelda fan game, and nobody ever thinks about exactly how much that entails. They're like a newborn child who wants to run before they can even stand.