I think a lot of it is that most topics outside of programming/tech have information locked up in books or other non-free things. For instance, I can't find a decent, in-depth article on how to make my own leather from deer hides. There are lots of general howtos, but many of them hint that the difficult and special knowledge is available in these books.

Maybe it's just that other knowledge areas don't just put every little thing on the web to be indexed and easily searched, or maybe I just grock programming well enough to read between the lines and follow implications, I don't know.