Because I'm at that delicious stage in my next novel, near the beginning, where I've got masses of material, tons of it, and I'm sifting through it, trying to make sense of it all.
One of the things I've realised a blog is really good for is trying out ideas. You know how it is with a novel - you have a great idea, it blows your mind, but when you come to write it down on the page it falls flat, doesn't seem a fraction as good. You feel deflated, the book is useless, you've suffered a major setback.
A blog doesn't work like that. You get an idea, half-formed, put it out there, people comment, you think about it, modify it, develop it and, hey presto, it's ready to go into your book. So the blog is like a nursery of ideas, a nice safe place to try things out, where they can grow safely. Or you can strangle them at birth, if need be.
A blog also helps to concretise your ideas. You're forced to think. To communicate. To connect. It's good discipline.
Yeah, on the whole we like blogs.