There are several reasons, actually.

         The original book / cover contained a subtitle: "A Tale of American Tyranny". It was removed from the second edition because some potential readers thought it implied that this was an anti-Tea Party book.

          The cover was improved.

          There were a few minor type-o's in the first edition. These were corrected.

          The first edition subtly implied that the main character blamed himself for, well, let's say, certain things that transpired in the story. It was important to make this point perfectly clear; not implied, but stated. Although the story stayed the same, a few lines were added here and there to clarify this.

          The first few pages of the book were re-edited. Again, same great story, but the re-edit better captures the reader's attention and imagination.