With blogger you don't need a database like you would with wordpress. Plus, blogger allows easier management and you don't need to know how to code to be able to put in things like Adsense, search bars, customization, etc.
Wordpress does give you much better management but you need to know some programming to be able to do it.
1. blogger doesn't need a mysql database 2. Blogger blogs get ranked better than blogs on other domains 3. The server load is on googles shoulders not yours 4. Better exppose because people can easily find your blog if they do a search on blogger. 5. Some people started with a blogger blog and they don't want to have to try to migrate their reader to a new location for the same blog in fear of losing some.