Personally I would say the number 1 reasons is a lack of focus and giving up too quickly (ok thats two reasons!). I see so many affiliates quit affiliate marketing because they dont make a sale within the first month. Many think that they dont have to do much work and results will come almost instantly - this is almost NEVER the case. You have to work hard and stay patient. Your first sale may take several months to arrive, but once it does you are on the way - keep working and results will come. Sometimes affiliates make sales in a matter of days, sometimes it can be months and months - but if you never give up you will succeeed. A lack of focus is also a big problem. Affiliates spend time building and promoting several sites but never concentrate on getting one or two sites working. Doing a little bit on many sites will make it even harder and longer to get sales. Far better to spend time building one or two sites up until they are working well before moving on to the next one.
I would say one of the biggest problems is, is that people try to promote in a too broad of a category. If you were promoting a real estate affiliate and trying to promote a national real estate service and expecting and hoping for a good return. Its all about finding a smaller niche within your affiliate and using long tail (more descriptive) keywords.
The truth is that once you find a niche for the product you sell, success will start flowing your way. It's all about finding the right niche.
"The secret of success is constancy of purpose." Disraeli. Most of beginners give up too quickly. It's true. And other part of people are too lazy for this job. You should very self-motivated for affiliate marketing.
it is not just marketing, if we are giving up on things in any area our life would fail. Giving up should not be in our vocabulary. When we make a move we must make move until we have exhausted all the avenues. And make sure before we start any move we have to study it carefully. Affiliate marketing is not about link and banner but more than that. Success has never been easy and will never be easy.
I agree with the first statement as I am guilty of either giving up too quickly or trying something different, before giving it a chance.
I have just written a blog post on this subject if anyone is interested: http://www.moreniche.com/blog/2008/marketing-strategy/what-makes-a-successful-affiliate-5-tips/
yah, its giving up too quickly or laziness. Also some people are just afraid to put in too much money to promote, thinking they loose it all so they just give up.