Hermione didn't know why she wasn't a great flyer. She had no fear of heights, so it wasn't that. Ronald used to tell her it was because brooms liked to believe they were in control, and since Hermione had to be in control all the time, the brooms would reject her. Hermione tossed aside that idea though. Brooms had no physical life to them, they were artificially charmed with magic so that they could be flown by wizards. Nothing more.