Because people are fickle, I guess. Your article needs to be geared towards some kind of product or product category that interests the viewer of your article. Even if they click and go to Amazon, they rarely buy the thing you were advertising.
Unfortunately, Hubpages doesn't seem to allow you any way of tracking individual Amazon ads. All you get is a lump sum of the clicks over your whole collection of articles, in your amazon affiliates account.
I do alright with Amazon, but it is very sporadic. I manage about 150 clicks a month (including my other websites), but only about 2 or 3 purchases of varying values. The Hubpages Ad program is much more consistent..and growing, for me at least.