As far I have seen many blogs in internet, I wonder why there is no blog which has subcategories in certain categories. For example, software subcategory located in computers category. They always use the categories apart so there is no subcategories. Why they don't make the subcategories ? Does it relate to SEO or maybe there is another reason why they do this ?
In some blogs it is impossible to create sub-categories. In others it is much easier to create a top-level category than a sub-cutegory. Very small number of people usually use advanced features. Sub-categories also mean more complex structure to maintain. Still I saw some blogs with subcutegories.
They probably don't realize subcategories are an option. As UseShots said, most people aren't aware of all the features their blogging platform has.
I use subcategories, but I think the other posters are correct that people don't know it's an option. I think it help visitors find information as articles get buried so easily in blogs.