My guess is it all has to do with how good the product is.

If I like it I'm going to give you and the product a rave review, a glowing testimonial for the sales page, help you refine the product for a re-launch or version 2,3 etc., even help you with some copy writing and suggested resources to help you make the best launch possible. If you offer good ROI percentages for affiliates I'll promote the heck out of it to my lists and via my marketing strategies.

If the product sucks, you can expect the exact opposite. If I bought the product and find out after spending good money that the product sucks I'll write appropriate reviews and plaster the internet ripping the product and author apart.

I just have to be honest about this as always.