#1 reason would be not having a mentor or teacher.
Internet marketing is a tough skill. Trying to learn how to be a successful IM on your own is like trying to be a successful piano player on your own. GOOD LUCK. It’s damn near impossible without structured guidance. Sure, there are always exceptions…but not everyone is born Beethoven.
Mentors aren’t just there to teach you. They also serve as a (successful) role model. Because otherwise…you’d (likely) be imitating people who aren’t even very good.