A lot of people -- my editor included -- think it's because of the varied content. Myself, I go for the second reason. My reasoning is based on the ratio of readers (over 300 a day) to sales (less than 20 a month).
Here is my superb theory.
Put EVERYTHING on the blog for FREE.
This way, at least I can see my time isn't wasted in writing. I was an idiot to think you'd pay nickels & dimes for months of work. Well, you may get your wish. I have nothing to lose, and yet again you, the readers win :(
My other option -- a serious one -- is too stop writing. This is something I don't wish to do, but if nobody reads what I write, is there a point to writing?