1. They've just worked on a similar book which didn't sell well.
2. They've just taken on another author who's writing in a similar style.
3. They're having a really bad day.
4. They're about to go on holiday but they've got to get through the slush pile before they can leave the office.
5. They've just been made redundant.
6. The imprint you've targeted has been cut.
7. The first reader is an 18 year old on work experience.
8. They just don't get it.
There are a million reasons why your work gets turned down which are nothing to do with you and what you have written. That's not to say you can get away with sloppy writing or editing, but just to point out that actually, sometimes it really is about them, and not you.