Maybe it has to do with how far one's donation goes.
For example, if I'm getting a generic, prewritten "thank you" email from a bunch of people that I don't know for contributing $10, then I don't see why it's worth bothering. This probably works fine for the Kickstarters with celebs involved with their projects, but not independent projects created by nobodies.
I'd probably be more willing to blow $10 on a campaign if I was going to get a small poster or something.