There are several reasons:
1: I don't like my full name being posted all over the internet.
2: The concept of circles does nothing for me. I have my friends and followers (who I enjoy) and do not need the contacts being expanded indefinitely.
3: I am not a fan of Facebook for similar reasons. Google+ is too much like Facebook.
4: I believe in the basics of KISS. (Keep it simple surely).
5: When I find things working well, I don't like things that break it.
Now, if Google would allow me to have my Followers widget back up, life would be good.