The difference between non-fiction and fiction are first the markets, and second the number of people who are doing it. I suspect there are more fiction writers. On top of the differences in numbers, fiction markets pay much less than nonfiction markets, unless you hit the jackpot. Professional pay rates are considered to be around 4 or 5 cents a word. That makes making a living as a fiction writer even tougher than making a living as a nonfiction writer.
Many writers are just more comfortable writing fiction.
So, when I say you can make a living writing for the web, I am referring specifically to writing nonfiction, although I am sure are there are some writers out there who make a living writing fiction for the web. There are tens of thousands of markets for nonfiction writers. Some of them pay only pennies per word. And still others pay according to the number of people who view your article. The reason I say that is that writing is because writing is a business just like any other business.