Earl Moore has a list of reasons why some people don't link.  I can add some additional reasons why I think a couple of bloggers I used to read don't link:

(a) I was picked on in high school and this is my revenge on the world;

(b) if I link to you, people will realize that you are right and I am wrong, and I like to feel right;

(c) for the first time in my life, I am at the top of the food chain in something and I like the feeling of ignoring people who wouldn't give me the time of day in the real world; and

(d) I'm pretending that my new media blog is an old media newspaper, and old media newspapers don't link to new media blogs.