There are several reasons for that:
- Blogging is shorter. When I write, I'm generally writing a story on the order of 8,000 words, or a novel on the order of 80,000. For a blog post, 800 words is long. My tendency to write longer does not help me write the succinct posts that blogging requires.
- Blogging has little, if any, chance for revision. My fiction has a rigorous, four revision process. A blog post may get a quick once over. I'm used to pushing through to the end of a story, then going back and making sure I get all the details right. One of the most important parts of that revision process is waiting. I can't go back and revise something right after I've finished. I have to wait some time, typically weeks, to get some distance from it, before I can look at it with fresh eyes. I can't do that with blog posts. I might be able to give it an hour or two, but that's all the distance I can manage before it publishes.
- Blogging has a different purpose. This one's so obvious that it's easy to overlook. In a story, I'm focusing on things like plot, characterization, and description. In blogging, I'm writing about facts and opinions, often in bulleted lists like this one. In a story I'm making things up; in a blog post, I need to get facts straight. Oh, they aren't entirely orthogonal endeavors. I need to get facts straight in stories too, when it comes to the real world parts. Blogging can often benefit from detailed description, and even a clear plot structure. Overall, though, they do have different purposes, and require different types of writing to meet those goals.