There's a lot of reasons why you could fail as an Affiliate, but there's 3 well known reasons why the majority of affiliates fail.
1. Bad Research: This is one of the most common reasons why affiliates usually fail. If you're simply jumping into a niche and building a campaign because you simply "think" it will work, it could end up biting you in the you know what. Doing your research on the niche and even keyword research will help you figure out which niche to jump into and which niche to stay away from, at least for now. You could possibly find a great product within a great niche if you take your time to do your initial research.
I've taught a lot of successful affiliates over the years and this is definitely one of the most common reasons others fail.
2. Bad Campaigns: Now if you've done your research, as stated above, you might find it strange why this would be here, but it's another very common reason for failure. See, even if you do great research, you could still create bad campaigns. These campaigns can be due to having a terrible site, terrible copy, terrible colors, bad CTA's (Calls To Action), or just bad traffic due to not doing proper keyword research. The last thing could be associated with number 1, but I'm keeping it here as a bad campaign utilizing PPC traffic, CPV traffic or even SEO can have bad keywords that are simply not relevant to the overall campaign.
What I find to be the most common denominator in bad campaigns is bad landing pages. When I find these LP's, they're usually a little bit of bad everything, from colors to wording, and even dead links in some cases! It's amazing by how bad some people get and it's sometimes associated with #3.
3. Laziness: Honestly, this is definitely what I see more than anything. People want to jump into Affiliate Marketing and they believe they can learn some stuff and just sit back and cash out. Well, really it's because they were fooled into thinking that this Business is easy. It's not that easy, just like your current/past job(s) aren't/weren't easy. Nothing in life is easy, even the things we believe are easy. There's a lot to learn in this business and there's some creative tools to help you out, but nothing is push button.
I spent nearly 2 years trying to help a friend who I always thought was a hard worker. But, like most others who get into this business, he was lazy and didn't want to do the actual work. That's too bad, because a lot of others who did do the work made a lot of money while he was busy browsing the internet and complaining about being broke.
If you noticed, we started from the beginning to the end. Most people start out by learning and they're really excited to get started (for the most part) and then create their first campaign, thinking it will make them rich. It's like buying a lottery ticket to some people. But they fail to do proper research. Then they move on and their campaign is now created.
Then comes the laziness, where they complain about how the training didn't work or the business isn't legit or anything else that will help them sleep at night, because they know they failed themselves. Affiliate Marketing doesn't fail people, people fail to do things right.
I can tell you, in all seriousness, that if you fix these issues and become persistent, you will make money online. There's more to it than learning and getting excited. Nobody becomes rich for no reason at all. It takes hard work!