Oh, lots of reasons. More reasons than I could ever list.
I should mention that I have been published traditionally. I have been a teacher, editor, competition judge, and screenplay story analyst. This isn't an either or thing. It's about a different venue, and a different paradigm.
So here are some thoughts, in random order:
*I like to blog. I like the web. I think the web is the new pulp magazine (especially if we remember that "pulp" didn't just refer to a specific kind of tough guy fiction we associate it with today, but rather to an extremely wide variety of things, in any and every genre, and non-fiction as well as fiction).
*I write things that ... aren't exactly what people expect. I learned a long time ago that my most ardent fans almost always start out thinking they won't like my work. It takes a lot of exposure to my work before people realize that I'm not doing what they think I'm doing. (And telling people that doesn't help.) So... it's in my best interests to expose people to as much of my work as possible. Especially in frequent small doses.
*There are many different audiences out there -- and there always has been. Library books, and piracy and lending and used books have always been a huge market, and it is NOT the same market as the folks who buy new hardbacks. Or buy new paperbacks, even. There is overlap, sure, but there is a very large audience who will never see your book if it isn't free or cheap. If they buy anything, it'll be books by authors they have become long hooked on, and can trust to be worth every penny. But a lot of those people can't even afford to buy much of those.
*Um, I hate to break it to you, but "traditional publishing" (that is, where you go to a publishing house and sell your soul so they will put your work out in book form) has been seriously broken for a long time. Signing a book contract these days is probably the worst thing you can do for your work or for yourself. It's one of the quickest ways to kill your book/series forever. Even the best contracts these days are downright unconscionable. If you're going to go with books for sale, go with self-publishing. You can always go back to traditional publishing once the traditional publishers get their act together, plus you've got learning to do anyway.
*There is just something fun about a serial. This is separate from my first point about liking the web. The serial form is all around us on television and comic strips and even certain kind of narrative column writing and radio. But in paid publishing, the serial has become very limited in scope. If you want to do this form professionally, AND do it in a way that suits the story, you will generally have to do it yourself, whether you want to or not.