Well, it could be that she heard a lot of spooky stories about the dark from her family or friends when she was growing up. These stories might have created an image in her mind that the dark is a dangerous and scary place. Additionally, if she has a vivid imagination, she might imagine all sorts of creepy creatures lurking in the dark. For example, she could be thinking about ghosts or monsters every time she's in a dark environment.
One possible reason could be a past traumatic event. Maybe she got lost in a dark place as a child, like a forest or a big, dark room. Another reason might be that she watched a really scary movie about the dark when she was young and it stuck with her. Also, some people are just genetically predisposed to be more fearful of certain things, and the dark might be one of those things for Jeannie.
Her fear of the dark might be related to a lack of control. In the dark, we can't see everything clearly, and for someone like Jeannie, this lack of visual information can make her feel very vulnerable. She may also have had a bad experience in the dark, such as being in a power outage alone and hearing strange noises. This kind of experience can really intensify a fear of the dark.