Reason 1: You don't want to be the one selling your book.
Traditionally, publishers do all the marketing for their authors' books. That takes resources, which is why it is hard to get a book accepted by a publisher. Lulu lets you publish your work without going through a publisher, but that means that the burden of selling and marketing your book falls to you. There are ways to do this, and the Web makes self-marketing easier than it has ever been. We do offer tips for marketing your book.
Reason 2: If you are looking to sell your book primarily through bookstores, there isn't much we can do to help.
While we do offer ISBNs for books published through us (which means that your book can hypothetically be ordered through bookstores), print-on-demand economics prohibit large discounts that bookstores require to stock books. Bookstores depend on the economy of scale available to large publishers and print distributors. Bookstores are only going to buy Lulu books when they are special-ordered by customers. The Web is a terrific way to sell books, as Amazon.com has demonstrated.
Reason 3: You're a very picky author.
In order to make our publishing system both affordable and scaleable, we have to limit choices about things such as paper and book size. We don't offer alternative paper selections or custom-trim books. We do offer download versions of books (e-books). For detailed specifications about the sizes we offer, see What paper will my book have for the cover and interior?
There are plenty of companies out there who will provide more options if you give them money. Lulu is the only publisher that offers you all that it does for free.