My first initial answer of "because people give up too soon" is not earth shattering...that's pretty well common sense, but I'd like to add a few things.
1) Far too many people view making extra money online or offline type programs as some type of scam or scheme...something that's borderline legit or doable. They fail to educate themselves on legit businesses and MLM type programs. They assume they won't make a dime even if they DO join one then lose interest quickly.
2) People fail to MASTER the game, the strategic game of getting more website traffic. They buy 1,000 guaranteed visitors from some site and get nothing in return. Little do they know they probably just bought "bots" and not real visitors. This means you need to learn proper sourcing to find legit sellers.
3) People never learn the fine art of listbuilding (which is crucial to ANY online business, PERIOD) If you can't build or have someone build a converting squeeze page then write action producing follow up letters for auto responders then no money will ever change hands.
4) People fail to learn proper blogging skills! Having a blog should be top priority in my opinion, because this is your home base where you will center your marketing efforts from. From here you can write good blog posts that can get indexed into the search engines thus bringing you lots of free traffic.
Providing you can write good SEO copy that is... Get a blog if you don't have one, learn about plugins, learn about all things blogging. You NEVER want to send your traffic to a stale outdated or cheesy looking company replicated website. You don't own this property so it's not always guaranteed to be there, plus you can't perform SEO or list build with it.
5) People never learn how to make marketing videos which are GOLD in terms or getting more leads
6) People never learn how to really communicate with others... they don't like to answer objections or deal with rejection. They feel icky by asking for the sale... and many other reasons.
7) They have a LOW tolerance for risk.. they don't want to spend money, because the lack the confidence that they'll see it back plus a profit. Business is risky, the owner of your company you work for took a risk or you wouldn't have that job. Learn to risk then move on if things don't work out.
8) PROPER training! People that come out of the corporate cubicle world or off the factory floor and go into a somewhat "alien" industry where not much makes sense to them. They pay a fee to join something ,get a link as it's called lol then told to talk to everyone they know. That's a recipe for failure right there.
These people need to be taught the basics of selling, telling, sifting and sorting... sometimes it can take years before some people find their groove.
I could go on, but I'll stop here- but my LAST point would be is that too many people believe the hype that's written in 1,000's of salesletters. ALL letters are written to sound great, but many are LOOSE with the truth.... BE TRUTHFUL when talking to leads! Getting them excited with lies is not right and won't be productive.
My 3 cents +