There is a single reason. There's always one reason, and it's always the same.
Lack of unique content: you haven't added anything to the sum of human knowledge.
Now, we could list all the bits that could be added to the sum of human knowledge, and point out that no, you haven't added each one of them. But so what? If you'd skipped all of them and come up with something else uniquely informative, we'd want to list the site anyway.
Or we could list all the ways in which you could eat someone else's dog food and regurgitate it in your web site. But so what? You could find a new way to plagiarize, or a new way of getting paid for doorway spam pages: and even though we'd never seen it before, we'd still not want to list it.
We could tell you the way we recognized your spam as spam, but we won't: we'd prefer people not waste time gilding their cowpatties before flinging them at us. And anyway, so what? Each editor has his own set of spam-detecting techniques -- but all of them are merely quick ways of recognizing that there's no point of looking for unique content on a site. In the end, before listing a site, we have to look for the unique content. If it's not there, ... it's not there. And that's the end of it.
So look at your site. What does it have we can't find elsewhere? Eh? Well, WE couldn't find it on your site either.
So that's the end of it.