Why the hell did it not catch on like FB?
Well, other than the idiotic "invite only" policy?
Well... it launched feature incomplete as compared to Facebook. Pages for example wouldn't be available for months after launch. Also, it took weeks to properly display Flickr links (in 2011, when Flickr still ruled the photosharing roost this was unnaceptable)... And when they did get it right, it was only kinda right. The thumbnails were very obviously downsampled and downsized - all the better to very visibly not compete with Google's Picasa service*.
The there was the stupid 'real names' mess just as G+ was starting to gain the smallest amount of traction.
Then there was emphasis on security and privacy over connection and sharing. (The latter being the whole point of a social network in the first place.)
Then there was whole tedious need to organize your Circles, G+ didn't work all that well (let alone as Google intended) 'out of the box'.
Etc... etc... Google kept shooting itself in the foot and giving people reasons not to switch or not to stay switched - and they did, in droves.
The basic problem is that Google is made up of geeks, not ordinary people - and they don't really grasp that their primary audience is ordinary people, not geeks.
This article is worth a read in that context, particularly points 3 and 4.
* Which a lame and half baked attempt to compete with Flickr.