Mostly network effects/inertia. And it didn't help that Google seemed to be quite pushy about getting people to sign up.
Someone I don't know added me to their circles. What the hell does that even mean? Do I now see whatever spammy crap they post? Do they see what I post? Why wasn't I asked for my permission? I've never met this person and I don't want to see their spam and I certainly don't want them seeing what I post. Who are they and what do they want? This sucks, I'm leaving.
As the sole means of creating a directed social graph, it's awful.
It means they want to see stuff you post, and/or post stuff that (if you circle them back) you will see.
>Do I now see whatever spammy crap they post?
What? No. That would be super dumb.
>Do they see what I post?
If it's public or if you post it to a circle that includes them.
>Why wasn't I asked for my permission?
Because they haven't been granted any special powers by virtue of adding you to a circle.
>I've never met this person and I don't want to see their spam and I certainly don't want them seeing what I post.
Then don't post publicly- choose a more limited audience to post to.
>Who are they and what do they want?
Go to their profile page and find out, if you care. But you probably don't- it's safe to ignore them.
This right here is where it goes from being a sane metaphor to an insane one. It's the phrase "circle them back" that has no meaning in human relationships, but is an all-important step in google plus.
I always understood the point of "circles" as how we privately define our personal world and our relationships with others, which may not necessarily be how those people view us. In comparison, I see Facebook's "lists" model less intuitive for people to understand and use.