If you want to get into specifics of exactly where a lot of internet marketers fail who do actually put in some work and try, it is traffic generation.
They think that if they build a "better" site than all the other crappy affiliate sites out there that Google is going to magically reward them with tons of traffic.
Or they think that a few posts on Twitter will get the ball rolling. It's not like there aren't millions of other Tweets every hour or anything.
They hear that content is king. They hear PPC is expensive. They are given fake maps to the mythical land of never-ending traffic.
You can stink at content creation. Your conversion rates can be lousy. You can be terrible at building an email list. However, if you can learn how to flood a website with targeted traffic, it is pretty hard not to make some money.