Two reasons.
First, it works. I posted some of my novels on Smashwords and Draft2Digital, and found very quickly that people aren't willing to pay for a novel they've never heard of. I had a book up for a few bucks for months, and while it got some views, it didn't have a single download. Meanwhile, the others which were free, got many times as many views and multiple downloads.
The second reason is philosophical. I go into it more in one of my Musings, but basically I believe that information should be freely available to everyone. Be it a novel, or a patent, or a piece of software, or a pharmaceutical formula. If we're all equally deserving, then we should all have equal access.
I still want to get published, and I'm still asking you to spread the word about my stories if you like them, but I will always have a free version of each book right here. That's one of the beautiful things about the internet- a free exchange of ideas. I mean, the internet is also loaded up with greedy, hateful ideas too, but those are free as well.
