Initially, it got a lot of buzz, people clamored to get invitations to join, and it became the fastest growing social network in history. But then... nothing.
And now, it's become fairly obvious that it simply hasn't found a place in people's lives.
The main reason given for lack of adoption is that people don't want to invest the effort needed to recreate the social network they've spent years building on Facebook.
That's true, but it's not the reason G+ did not take off.