You know that your site has unique value, but the webmaster you’re pitching doesn’t. If he/she knew that value, you wouldn’t have to be pitching them because they would likely be linking to you already. And the value you can provide for site A can differ from what you can provide for site B. This makes blanket pitches highly ineffective.
I’m talking about value proposition, a crucial component of any marketing campaign. If you don’t explicitly state the value of what you have to offer to your audience, your audience is not going to see it for you.