There are many reasons why you should use WordPress Open Source to create a Blog. Here are the important ones (Not any Particular Order):
1. It’s very easy to install.
2. It is the easiest to understand, the documentation is in layman terms. The step by step procedures are outstanding.
3. Once installed, it does everything for you. Installing Plugins are done just by clicking.
4. It is constantly being updated.
5. It is the Number “ONE” open source software for Bloggers. (Checkout the comments)
6. There are hundreds Plugins and most of them are updated. Whenever any Website or Blog enhancement tool is released, a WordPress Plugin is created so you can simply add it to your Blog.
7. Most Online Webhosting companies support WordPress.org open source.
What else can I add here?