I can only answer for me, I've been a member for three years, and I only have ten hubs. Several reasons I suppose, but mainly:-
If I wish to make money from writing, I have, and can do it easier writing for print magazines.
Hub pages and its associates seem to be going out of their way to make it difficult for hubbers outside of North America to make revenue.
In the early part of 2011, I suffered a stroke, I'm in the middle of the recovery process.
I'm an old age pensioner, and any thing I earn will reduce my pension, so it's not worth working for the government. So I have effectively retired from making money from writing.