First, it didn't make sense that people would quickly leave an established network (FB) to join a new empty network. At this time, most folks who wanted to join a network had finally joined FB. It took a lot of time and momentum for FB to reach that point. Google+ didn't offer anything fundamentally different from Facebook: it was very similar, just with far fewer people to talk to.
Second, Google+ wasn't cool. Most successful non-niche social networks seem to grow from young people first: young people enjoy having an online space that's separate from older friends/family. Then, as the network becomes more popular, older folks join. This is loosely the trend we've seen with Facebook and Instagram, and to a lesser extent with Snapchat and MySpace. But, to get young people to join, you have to offer something cool and/or unique. Google+ wasn't very cool or unique: it seemed to target everyone at all once.
G+ failed for other reasons too of course, which others have listed in the comments here. But I think the biggest reason it failed was because there wasn't any compelling reason to use the network.
(I'm a Googler, but my opinions are my own)