1) It is FREE!*
2) It has a wonderful community. People are social in this community and love to read and comment on blogs of similar interests.
3) It's Google. Who is smarter than Google?
4) Ad Revenue. You can use AdSense by Google in Blogger. You cannot do that in WordPress.com (You can in WordPress.org but it will cost you money). I've been in the ad business all my life and can't image not finding a revenue generating source for content.
5) Support. Google helps and forums are there for you to use at no charge and answers usually come quickly. Who doesn't get stuck on technical issues?
*Blogger and WordPress.com are both FREE but I do recommend you get your own URL - domain name which does cost money. WordPress.org does cost money to host and sign up but does offer plug-ins and options. (That may be my next experiment).