The point is why she can see...
...and not why they can't.
People still wonders what's the reason of the blindness, and, although with the movie being have made with a difficult script, you'd find out.
I wont lie. If you haven't read the book, probably it wont make any sense, unless youve got A LOT of imagination. The flaw in the movie is actually not taking in consideration those people. Still, for me having read the book, it felt like they missed the direction, and the reason is that the story is way too big. The simbology is too much, and is too difficult to made. Lynch is not my man, but... I Think he'd would have done it better. Still, and despite all these, is a very solid film.