Wordpress is easy to install if you're already familiar with setting up a LAMP stack. A lot of people who just want to blog are not. It's easy to get running and install themes, but honestly it's almost a full-time job to keep it running right. You have to keep up with security updates and deal with confusing config options for custom domains. wp_config, Apache, and MySQL are not something you can use casually.

Also, performance is terrible unless you cache aggressively, which most people don't because it's a lot of work, even with W3. The nice thing about real hosted solutions is they can handle extra load if it happens.

I get why it's popular. The blogging interface is very good, but trying to setup/maintain a server makes me want to pull my hair out.