Why do lit-ah-rary types look down on SF

So what is it about science fiction that causes "literary" types to look down upon it? Like any genre, SF has its bad and good. No scratch that, like any writing, there is both bad and good. I've read plenty of unreadable "literary" fiction. But SF seems to get more derision than other forms of genre writing, perhaps unfairly. Many important books are SF.

Yet, try to get Margaret Atwood to admit she writes it; yes, she writes SF (though anti-technological, like Michael Crichton) and...