1. You don’t get enough traffic.
2. You get the wrong traffic.
3. You promote the wrong product to the wrong people.
4. You give up too soon.
5. You don’t understand your product.
6. You don’t rely on your affiliate manager.
7. You can’t build a proper website.
8. You don’t research your niche.
9. You don’t have a niche.
10. You only promote one product.
11. You don’t know how to build a landing page
12. Your copywriting sucks
13. Your design isn’t appealing
14. You overloaded with banner ads and drove prospects away
15. You built your site in flash loser
16. You didn’t insert your aff code properly (ha ha!)
17. Your testimonials are teh gay
18. adsense overload
19. You lose your customer with expired Aff product links
20. Your page loads far too slowly.
21. Your images don’t load.
22. Your merchant is offline.
23. Your merchant doesn’t pay his affiliates.
…

You can check out the full list Here

If you are into affiliate marketing and wondering why your profits suck you should really check it out. If your profits don’t suck check it out anyway or you will fail at life and die alone in a dirty one room apartment with a toothless hooker eating your last slice of pie.