Just consider a few of these things:
- The release date of your book is not based on when you want the book to come out, but a schedule of other authors all with the same editor, publisher and line.
- Weather and other natural problems will play a factor. Super Storm Sandy has truly made all of the publishing world play catch up.
- Trends shift in what the readers like. One day Zombies are hot and the next day they're not. You learn this well if you have girls living in your house that are fashion conscious.
- Other projects get in the way of your edits. Your editor might have every good intention of getting your book's revisions to you and then another book has a bigger crisis.
- There are only 24 hours in the day. Editors and agents may say they will get to your book ASAP but sometimes they too need a break from reading (DO YOU HEAR THAT SUSAN! TAKE A BREAK!)