I've said it before
, but I'll repeat it here: Google didn't know how to capture public interest at the time.
I remember when Google+ first appeared as an "invite only" service. That was just before Facebook made the huge blunder of putting members' profile photos in ads for any pages they "Liked," suggesting an endorsement. A lot of people everywhere got really angry at Facebook about "faces on ads," and even threatened to leave Facebook because of it.
That would have been a great opportunity to open up the Google+ service to everyone, seize the opportunity when people wanted to abandon Facebook. But Google+ remained invite-only. Only a few people could get new accounts.
Over the next week, pretty much all you saw in the news was how people wanted to leave Facebook because of the "faces on ads" thing. What an abuse of privacy! You're stealing my image to sell products! There were a bunch of petitions for Facebook to undo the new "faces on ads," or else they would delete their Facebook accounts. The only problem was that there wasn't a viable alternate social network out there. Twitter wasn't really a replacement for how most people used Facebook.
And Google+ still remained invite-only. By then, a few people I knew had accounts, but had run out of invites to share. So few others could get in.
After a few weeks, Facebook decided to calm the storm, and undid "faces on ads." And as expected, people stopped freaking out about Facebook. After another week, even the tech websites stopped writing about "faces on ads."
And finally, Google+ went "live." Anyone could join. I had an account, but few of my other friends bothered to sign up. Why? Because they were still using Facebook, they got over the "faces on ads" fiasco. Without other people to share with, Google+ failed to gain critical mass.
Google+ failed because they didn't know how to respond to the opportunity that Facebook gave them.