JK Rowling: Because I had glasses all through my childhood and I was sick and tired of the person in the books who wore the glasses was always the brainy one and it really irritated me and I wanted to read about a hero wearing glasses.
That always pissed me off too. It was like a rule of fiction - specs=swotty dork. Now, I had elements of swotty dorkishness about me, of course, but I really craved a heroine (or hero) who wore glasses. I would have loved Harry's bespectacled state as a kid.