The reason is - Blogger is free CMS as well as a free hosting service. If you want to host a subdomain on free hosting service, they do not support adding a subdomain or support only limited number of subdomain. Blogger is easy to write Blogs which are related to documentation and service usage guide that does not require SEO focus. The choice is up to you. If you have enough money or unlimited hosting plan, you can create a separate subdomain and use for your purpose.