1. There is an overt link to an affiliate sign up page. So your potential customer could simply become an affiliate and purchase the product through his own link.

2. There are many external links which distract the visitor from reaching the order page.

3. The merchant captures your commission, for example

There are offers to join a mailing list or claim a free report: you should get credited for any sale ensuing from these leads but an unscrupulous merchant may follow up and make the sale via his own link.
Similarly the merchant may use a pop-up upon exiting a site to convert a leaving visitor into a sale for which you do not get the credit.
4. Your product is just one of many unrelated products on a page – a put-off for the potential customer.

5. Remember ClickBank mentioned previously ?

If you are a ClickBank affiliate and the merchant offers multiple payment options (such as Paypal) then if ClickBank is not chosen you do not get the commission.